In an impact crusher, a rise of an advancing-retracting portion (65) of a gap adjuster (60) caused when a rebound plate (33) touches a rotor (32) or the rotation amount of the rotor (32) is detected to determine a zero-point position of the rebound plate (33) is determined based on the detected result, so that the zero-point position can be securely determined since it is not necessary to touch and vibrate the rebound plate with the rotor rotating at a high speed, and since the rebound plate (33) is not vibrated, the zero-point position can be securely determined without being influenced by the abrasion of the rebound plate (33) and the impact plate (322), thereby accurately adjusting gap (C).