While a check ring of a screw of an injection molding machine is in open state, an absolute value (|Δ resin pressure/Δ rotation amount|) of a variation of a resin pressure per unit variation of the screw reverse rotation becomes greater than a predetermined reference value. However, after the check ring is closed, |Δ resin pressure/Δ rotation amount| becomes equal to or less than the reference value. From this, detecting if the |Δ resin pressure/Δ rotation amount| during reverse rotation of the screw, it is possible to determine that the check ring is closed or not. As a result, it is possible to obtain a screw reverse rotation amount which is optimal for reliably closing the check ring.