The present invention relates to a camshaft that opens and closes an exhaust valve in an exhaust port and an intake valve in an intake port of an internal combustion engine, in which the intake valve alternately opens and closes the intake port, and the exhaust valve alternately opens and closes the exhaust port, when a piston reciprocates in a cylinder. The camshaft comprises, as a cam that opens and closes the exhaust valve and the intake valve, a ball cam whose protrusion amount changes according to rotation of the camshaft, wherein the camshaft has a double structure consisting of an inner shaft and an outer shaft provided in a manner that the inner shaft is helically displaced with respect to the outer shaft around an axis of the camshaft according to a rotation speed of the camshaft, and the ball cam is accommodated movably in a guide groove provided in the inner shaft and protrudes from the outer shaft, and a protrusion amount of the ball cam from the outer shaft changes when the ball cam moves in the guide groove due to the helical displacement of the inner shaft with respect to the outer shaft, wherein, on a contact surface between the outer shaft and the inner shaft, a helical screw is provided, and at a bottom of the outer shaft and the inner shaft, a pressing spring is provided.