PURPOSE: To obtain stable detecting output and to facilitate the handling of a device.

CONSTITUTION: At the time of executing no injection, passages 16, 17 for detecting pressure and the differential pressure detecting means 30 are cut off and also, by always spouting the air to the passages 16, 17 for detecting pressure by air purge means 20, this passages are cleaned. At the time of executing the injection, the air spouting into the passages 16, 17 for detecting pressure is interrupted, and also by communicating the passages 16, 17 for detecting pressure with the differential pressure detecting means 30, the differential pressure between the pressure P1 at the inlet part 14 of a venturi flow passage 13 and the pressure P2 at a stroke part 15, is detected and this flowing is grasped to detect the gas venting condition in the die 73.