A mechanical adjuster that automatically increases or decreases or decreases valve clearance is provided that includes: a housing (12) having threaded portions (24, 26); a threaded member (14) thread-coupled with the threaded portion (24); a threaded member (16) connected to the threaded member (14) via mutual rotation prevention-couplings (32, 38) and thread-coupled with the threaded portion (26). When a load acting on the threaded member (14) is equal to or less than a valve closing load, valve clearance is decreased by an integrated rotation of the threaded members (14, 16). with a slip torque of the threaded member (14)>a self-locking torque of the threaded member (16). When a load acting on the threaded member (14) is close to or exceeds a valve opening load, valve clearance is increased by an integral rotation of the threaded members (14, 16) with the absolute value of a slip torque of the threaded member (14)>a self-locking torque of the threaded member (16).