At the Labor Bureau of the city where you work at.

Thanks, Happy! Is it really necessary to have Working permit in my possession? 

School told me, Working permit is at the Labor department.

If you have a Residence Permit that issued from a Z visa, and if you have a Foreign Expert Certificate, then as a matter of due legal course, your employer already caused a Work Permit to issue on your behalf from the Ministry of Labor.  In terms of what it looks like physically, it is an A5 size of white paper, with a black border around in, containing information concerning your employer and your own personal details and it certifies that your employer has the legal right to employ foreign nationals. 

The reason that I say that your employer has this Work Permit in his or her possession is that no Foreign Expert Certificate and no Resident Permit will issue without it.

In order to avoid any problems, I do insist on having the originals of all my documents, and my employer can, if wanted, keep copies as required for his records.  I agree with the first reply, if you have your FEC and your residence permit, then somewhere your employer must have a work permit.  Should you have it too?  In my book it is a definite yes, because the work permit is the legal document that allows you to work in China.

Working permit is written entirely in Chinese, with red official stamp on the bottom R.

I have Working permit in my documents file, since I applied for Z visa in Hong Kong.

Thanks for your answers. I have to do some characters learning