What could it be?
更新1:
Please stop saying it's cruel about me putting him in the cage. Firstly, he is out from his cage MOST of the daytime, playing, running all over. At night time when we all sleep we have an alarm in the house so we need to put him inside the cage. The cage is not THAT small. It has a foodbowl/water and a litter box. He's inside usually at 11pm-5am and 6am he's out until 11pm. Relax. I play with him all the time. He's happy and very healthy. I just don't know what caused the death.