Provided is a mechanical lash adjuster including a plunger, a housing put into thread engagement with the plunger in an axial direction to form a thread engagement portion, the plunger engaging member fixed and retained in a circumferential direction of the thread engagement portion, and a coil spring urging the plunger in a direction opposite to a urging-force acting direction of a coil spring. A lead and flank angles of threads forming the thread engagement portion are set such that when an axial load acts on the plunger in either of a plunger extension and contraction directions, the plunger is made self-sustaining, and when the plunger swings by an amount corresponding to a backlash due to a lateral load, the plunger slides and rotates at the thread engagement portion to move in an axial-load acting direction.