A return spring is formed using a compression coil spring that is installed so that its axial direction is an up-down direction. The return spring is provided in a high-pressure chamber, and powers a plunger in an upwards direction. A coil diameter of the return spring varies along the axial direction. When the return spring is in a maximally compressed state, an axial direction dimension (up-down direction dimension) is smaller than a dimension that is a thickness of a spring wire multiplied by the number of coil turns. Hence, an overall height of a lash adjuster A can be reduced in comparison to when a return spring with a constant coil diameter over an entire length is used.