Will a Mongolian passport holder need a visa to China?

Hello,
I am going to Chengdu from Tokyo for 3 days. I have an Mongolian ordinary passport. The Mongolian side confirms me that I do not need entry permit if it is less than 30 days. However, the Chinese side in Tokyo says I need entry permit. Why is it so? I have travelled to China many times and I never needed entry permit. Even a month ago, someone I know went to China without needing entry permit.