A valve mechanism comprising: a cam rotating in conjunction with rotation of an engine output shaft; a shaft end portion of a valve urged in a valve closing direction by a valve spring; a power transmitting member interposed between the shaft end portion of the valve and the cam to transmit a pressing force of the cam to the shaft end portion of the valve as a valve opening force; and a mechanical lash adjuster linked to the power transmitting member and adjusting a valve clearance between the cam and the power transmitting member, wherein the mechanical lash adjuster includes a plunger that is brought into contact with the power transmitting member and to which the pressing force of the cam and an urging force of the valve spring are transmitted through the power transmitting member, and a plunger engaging member that is put into thread engagement with the plunger to form a thread engagement portion cooperating with the plunger to extend and contract the plunger based on rotation relative to the plunger and that is retained non-rotatably in a circumferential direction of the thread engagement portion, characterized by a compression coil spring associated with the plunger and the plunger engaging member and urging the plunger in a direction in which the power transmitting member comes into contact with the cam, and a torsion spring associated with the plunger and the plunger engaging member so that the plunger is urged in a relative rotational direction for extension from the plunger engaging member, wherein the thread engagement portion has thread ridges of threads constituting the thread engagement portion whose lead angle and flank angle are set such that when a load (F1, F2) acts on the plunger in either of extension and contraction directions of the plunger, slide rotation of the plunger relative to the plunger engaging member is suppressed in the thread engagement portion by a friction torque generated in the thread engagement portion and that when a lateral load (T1, T2) acts on the plunger to cause a swing of the plunger relative to the plunger engaging member, the suppression of the slide rotation is relieved. a plunger that is brought into contact with the power transmitting member and to which the pressing force of the cam and an urging force of the valve spring are transmitted through the power transmitting member, and a plunger engaging member that is put into thread engagement with the plunger to form a thread engagement portion cooperating with the plunger to extend and contract the plunger based on rotation relative to the plunger and that is retained non-rotatably in a circumferential direction of the thread engagement portion, characterized by a compression coil spring associated with the plunger and the plunger engaging member and urging the plunger in a direction in which the power transmitting member comes into contact with the cam, and a torsion spring associated with the plunger and the plunger engaging member so that the plunger is urged in a relative rotational direction for extension from the plunger engaging member, wherein the thread engagement portion has thread ridges of threads constituting the thread engagement portion whose lead angle and flank angle are set such that when a load (F1, F2) acts on the plunger in either of extension and contraction directions of the plunger, slide rotation of the plunger relative to the plunger engaging member is suppressed in the thread engagement portion by a friction torque generated in the thread engagement portion and that when a lateral load (T1, T2) acts on the plunger to cause a swing of the plunger relative to the plunger engaging member, the suppression of the slide rotation is relieved.