A tappet clearance automatic adjusting device comprising an adjusting unit for advancing/retracting an adjust screw from the forward end of a rocker arm and adjusting the projection amount, a torque detecting section for detecting the torque by rotating the adjust screw, and a control mechanism section for controlling the adjusting unit based on a torque value measured at the torque detecting section. The control mechanism section starts retracting the adjust screw when a valve is opened and continuously measures the torque value obtained when the valve is closed. A position, corresponding to time when variation in the filtering value obtained by smoothing the torque value through moving average is a threshold value or smaller, is detected as a reference point and the adjust screw is retracted from the reference point by a set amount.