Relieving letter is must for Z visa?

Hello, i am currently employed with an Indian company. I got offer letter from China and now i need to apply Z. My employee at China suggested to resign after i get it. I checked with a consultancy here at India and they said the embassy will not give Z until you show the relieving letter from present company. Is this relieving letter required? i mean once i get the Z i will resign from my company and then travel to joint he other company.