A valve mechanism for an internal combustion engine, comprising: a cam that rotates in conjunction with rotation of an engine output shaft; an engine valve that is lifted and opened through rotation of the cam; a variable lift amount mechanism arranged between the cam and the engine valve to continuously vary a maximum lift amount of the engine valve; and a lost motion mechanism, wherein the lost motion mechanism absorbs a lift amount of the engine valve to maintain the engine valve in a closed state, wherein the lost motion mechanism includes a hydraulic lash adjuster arranged between the cam and the engine valve to automatically adjust a valve clearance of the engine valve, an urging member capable of contracting when the lash adjuster receives drive force from the cam, and a switch portion for switching an operation state of the lost motion mechanism between a permitting state for permitting contraction of the urging member and a prohibiting state for prohibiting the contraction, wherein the switch portion switches the operation state of the lost motion mechanism to the permitting state when the lost motion mechanism maintains the engine valve in the closest state, and a maximum contraction amount of the lost motion mechanism at the time when the lost motion mechanism is switched to the permitting state by the switch portion is set to such a value that the lost motion mechanism absorbs a predetermined lift amount that is smaller than a maximum value of the maximum lift amount of the engine valve. a cam that rotates in conjunction with rotation of an engine output shaft; an engine valve that is lifted and opened through rotation of the cam; a variable lift amount mechanism arranged between the cam and the engine valve to continuously vary a maximum lift amount of the engine valve; a lost motion mechanism, wherein the lost motion mechanism absorbs a lift amount of the engine valve to maintain the engine valve in a closed state, wherein the lost motion mechanism includes a hydraulic lash adjuster arranged between the cam and the engine valve to automatically adjust a valve clearance of the engine valve, an urging member capable of contracting when the lash adjuster receives drive force from the cam, and a switch portion for switching an operation state of the lost motion mechanism between a permitting state for permitting contraction of the urging member and a prohibiting state for prohibiting the contraction, wherein the switch portion switches the operation state of the lost motion mechanism to the permitting state when the lost motion mechanism maintains the engine valve in the closest state, and a maximum contraction amount of the lost motion mechanism at the time when the lost motion mechanism is switched to the permitting state by the switch portion is set to such a value that the lost motion mechanism absorbs a predetermined lift amount that is smaller than a maximum value of the maximum lift amount of the engine valve. a hydraulic lash adjuster arranged between the cam and the engine valve to automatically adjust a valve clearance of the engine valve, an urging member capable of contracting when the lash adjuster receives drive force from the cam, and a switch portion for switching an operation state of the lost motion mechanism between a permitting state for permitting contraction of the urging member and a prohibiting state for prohibiting the contraction, wherein the switch portion switches the operation state of the lost motion mechanism to the permitting state when the lost motion mechanism maintains the engine valve in the closest state, and a maximum contraction amount of the lost motion mechanism at the time when the lost motion mechanism is switched to the permitting state by the switch portion is set to such a value that the lost motion mechanism absorbs a predetermined lift amount that is smaller than a maximum value of the maximum lift amount of the engine valve.