PURPOSE: To measure easily generated gas by weight of a material and gas quantity, by a method wherein a cavity of a mold is made into one where its flow resistance is little and it does not have dwell of gas, the inside of the cavity is made at atmospheric pressure, molten resin flows within the cavity by casting the molten resin into the cavity and the generated gas and residual air within the cavity are measured.

CONSTITUTION: A spirally shaped cavity 11 whose sectional form is almost semicircular is provided in a mold 1 and a molten material P is constituted so that its flow resistance is little and it does not have dwell of gas G. A casting port 12 of an injection machine S is provided on a starting end 11a of the cavity 11 and an exhaust path 13 of the gas G is communicated with a final end 11b of the cavity 11. A gas quantity measuring apparatus 2 is joined to the cavity 11 which is made into atmospheric pressure. The material P cast into the cavity 11 is measured and the generated gas G is measured by a scale of a measuring cylinder 23. Therefore, a generating quantity of the gas G per weight of the material P can be known and it is made that a scale changing based on the residual air quantity is integrated.