an upper side by a cam nose 72 the plunger 60 is moved to a lower side relative to the body 20 by being pressed by the rocker arm 90 the working fluid flowing into the high pressure chamber 26 is compressed and a pressure in the high pressure chamber 26 is elevated in accordance with elevation of the pressure in the high pressure chamber 26 a small amount of the working fluid in the high pressure chamber 26 is moved up to meander through the gap between an inner peripheral face of the cylinder portion 22 of the body 20 and the outer peripheral face of the peripheral wall 62 of the plunger 60 passes through the flow path 51 of the spacer 50 and the small clearance 40 thereafter made to flow into the lower pressure chamber 65 by way of the plunger hole 68 an entire length of the lash adjuster 10 is shortened slightly by an amount of the working fluid flowing out from inside of the high pressure chamber 26 further by elevating the pressure in the high pressure chamber 26 the body 20 and the plunger 60 are integrated to be rigid and the lash adjuster 10 functions as a fulcrum of the operation of the rocker arm 90 when the cam nose 72 is brought into a state of being directed to the upper side from a lowermost point in accordance with rotation of the cam 70 a force of pressing the rocker arm 90 is nullified and the plunger 60 is pressed back to the upper side by the pressure in the high pressure chamber 26 and the urge force.