PURPOSE: To prevent defective filling by making cutting operation unnecessary, by a method wherein a fluid passage whose one end communicates with an air rich part of a cavity is formed on a split surface of a mold, an overflow chamber is formed by communicating with the other end of a fluid passage and an air passage communicating between the overflow chamber and the outside is formed.

CONSTITUTION: A fluid passage 5 is formed into a part where one end communicates with an air rich part 4 of a cavity 3, in a split surface 1 of a mold and an overflow chamber 6 is formed by communicating with the other end of the fluid passage 5. The overflow chamber 6 is formed at a lower position than that of the fluid passage 5 and the same is a chamber where a resin material 7 which has passed away through the fluid passage 5 is accumulated. Furthermore, an air passage 9 communicating with the outside is formed by communicating with the overflow chamber 6. A valve 10 is arranged in an air passage 9 and the valve 10 is opened when air pressure within the overflow chamber 6 exceeds an establish value. A pressure sensor 11 is arranged on a downstream side of a valve 10, the pressure sensor 11 detects opening of the valve 10 and applies stopping command signal to an injection molding machine 12.