In a decompression unit including a decompression arm, a decompression unit for internal combustion engine is provided in which the followability in the opening-and-closing operation of the air intake valve or the exhaust valve with respect to the cam at high-revolution speeds is satisfactory without increasing the equivalent inertia weight of the valve driving system, and for lowering the cost. A decompression unit includes a first decompression cam that takes the operating position when the number of revolutions of the engine is not more than the predetermined number of revolution during startup. A second decompression cam is provided that takes the operating position by manual operation. A decompression arm includes first and second abutting portions that abut against first and second decompression cams, respectively. An adjusting screw is provided for pressing the rocker arm and a resilient member for bringing the first abutting portion of the decompression arm into abutment against the first decompression cam. The decompression arm pivoted by the first and the second decompression cams pivots the rocker arm via the adjusting screw to open the exhaust valve.