To provide a gas emission preventer having a high reliability of such a structure as not shutting off in a momentarily overflow condition which is not abnormal and shutting off in an overflow condition which is continued for the specified period of time.

A shutoff valve mechanism 3 held in a body 2 having flow passages 2a and 2b taking the open position with the aid of a retention spring 16 makes shutting-off with gas pressure and is equipped with a case 5 held by the retention spring 16, an auxiliary rod 6 fitted with an auxiliary passage 13 attached to the front of the case 5, and a ball valve 8 installed in the case and stored inside a guide member movably for opening and closing the auxiliary passage 13, wherein if the gas flow becomes a temporary overflow condition, the case 5 moves to shut the passages 2b and 2c to allow the gas to flow from the auxiliary passage 13, and in the case the overflow condition continues, the auxiliary passage 13 is choked to generate the shutoff condition.