Why is it that Japan never declared war on the USSR once Germany
invade, since Hitler declared war on the US after Pearl Harbor ?

Here's one person's opinion:

"Why didn't Japan attack the USSR in 1941?
Scotty (Dec 3 2003 - 03:19)
 
The Italians helped with the invasion of Russia, as did other Axis
minor countries (Hungary, Slovakia, Romania, Finland), but not Japan.
This is probably because Japan and Russia had already fought in 1938,
at Khalkin Gol (called Nomonhan by Japan) in Mongolia. During this
fight, the Japanese discovered that the Russians of 1938 were
different from the Russians of 1904-05. The Red Army brought a horde
of tanks and artillery to the fight and totally wiped out several
Japanese divisions, who when surrounded, predictably fought to the
last man. The Russian air force was also very active, but the Japanese
air force came off well, as the Nakajima Ki-27 was better than the
Polikarpov I-15 and I-16s, and the Japanese pilots far better trained.

After the 1938 experience, Japan was not eager to fight Russia again.
Russians had been very tough, stubborn opponents in 1904; Japan's
general Nogi lost both his sons trying to take the large, 203 meter
hill outside Port Arthur. What defeated Russia then was material
shortages of every kind and incompetant leadership at high levels.
Stalin's industrialization program (like the steel mill complex at
Magnetogorsk) ensured that the Red Army would have no shortages of
industrial products the second time 'round."

http://www.japantoday.com/e/?content=bbs&order=msg&author=Scotty&page=7

The answer is easy to explain, especially when considering Japan's
actual military capabilities. Japan's armies were vastly unable to
match the capabilties of any Great Power who brought their resources
to bear against her, and thus the example of Japan's 'skirmish' with
Soviet forces in Mongolia should be considered one of the main reasons
Japan did not declare war: Japan's military organization was based on
fighting poorly-armed guerrilas and infantry formations across the
plains, with small tanks and infantry. The Soviet style of fighting
was massively  tank-based, and along with her superiority in numbers,
the Japanese would have recieved a major thrashing. Germany could not
withstand the force of Soviet arms, so how could a much smaller
country, totally unequipped to fight the USSR, such as Japan, do any
better?

There is one final thing to consider: At the end of the war, after
Germany had been defeated, the USSR finally declared war on Japan.
Manchuria was immediately liberated, and the mass formations of
Japanese troops (still on the continent) were obliterated. Because
Japan signed for peace before Russia could take any more territory,
only the Kurile islands were rended from her grasp. It is likely that
Kyoto would have been a Soviet sector without these bombs.

Hi
I think that the answer is already up there, but I would like to add somethings.
-I found out that Japan was going to fight USA and other allies even
after the bomns of Hiroshima and Nagasaki.
-Japan surrendered two weeks after the USSR declared war on Japan.
-And I think that Japan surrendered only after the USSR declared war
because: the USA and other allies were too distant from Japan and
could take months to get there, and in that time, Japan could build up
some forces in China and Machundria. But USSR forces were near the
Islands of Japan and could take over in less than one week. Also
because USA and other allies would only bomb and probably not conquer
Japan, but USSR was going to expand and take over the wole Japan's
territory if Japan didn't surrender.
-So, Japan knew that only USSR would take their territory and not to
do lose it's territory and power, Japan never declared war to USSR.

There also would be another question: Why didn't USSR declare war to Japan?
-USSR had already too many problems with the Western front (Germany,
Italy, Austria, Finland, and other Axis powers), and didn't want more;
but if needed USSR could send troops to conquer Japan.

I don't know if this is what you were asking for, but this is my contribution.