In a lash adjuster of a valve gear which employs a serration-shaped thread mechanism, the formation of a tribochemical reactive film is suppressed by using as the materials for its adjuster screw and nut member (or the materials for their thread surfaces) materials that, even if FM oil is used, the friction coefficient will not extremely decrease. The nut member is provided on the underside of an end plate of a lifter body. The adjuster screw is threadedly engaged in a threaded hole of the nut member. The adjuster screw is biased by a return spring. The female threads of the threaded hole and the male threads of the adjuster screw are serration shaped. One or both of the nut member and the adjuster screw, or the pressured thread surfaces of one or both of them, are formed of a material that will not react with oil additives of FM oil to suppress the formation of tribochemical reactive film, thereby stabilizing the operation of the lash adjuster.