To provide a vacuumizing method of a mold cavity which utilizes the air pressure of an existing compressed air pipeline, a simple vacuum means built-in mold utilizing a Bernoulli law, a simple vacuum device built-in venting device, a simple vacuum device and an ejector.

In the vacuumizing method, the simple vacuum means built-in mold and the simple vacuum device built-in venting device, the venting passage 6 communicating with the mold cavity E and the simple vacuum means 2, which is provided to the outlet side of the continuous venting passage 16 of the venting device G communicating with the venting passage and utilizes the air pressure of an existing compressed air source F to suck air H from the venting passage, are provided. The simple vacuum means is equipped with at least one of an ejector part 3 which utilizes at least a Bernoulli law and an on-off valve 4 opened at the time of mold clamping of a mold but closed at the time of mold opening of the mold and constituted so as to suck air from the mold cavity.