When the screw moves forward, resin backflow occurs. The backflow acts on the screw, applying torque to the screw. During forward movement of the screw, screw torque is produced as shown in FIG. 3A if the check ring is not worn, or as shown in FIG. 3B if the check ring is worn, because wear increases the backflow and delays the closure of the check ring. The magnitude of the peak screw torque and the associated time and screw position vary depending on the presence or absence of wear, so wear of the check ring is estimated from variations in these physical quantities. The state of wear of the check ring, screw head, check seat, and other parts inside the barrel can be estimated.