To provide a valve structure for an internal combustion engine capable of smoothly operating a changeover member such as a changeover piston at a predetermined timing.

A hydraulic passage 11d is formed in a cam shaft 11 having adjacent cam 11b for high rotation and cam 11a for low rotation, an intermittent oil groove 11f is formed in the periphery of a journal part 11c of the cam shaft 11, and an oil passage 12b is formed in a journal receiving part 12a of a cylinder head 12. The intermittent oil groove 11f and the oil passage 12b are communicated when the cam shaft 11 is in a predetermined turning range, and the communication is broken when the cam shaft 11 is in the other turning range. Oil pressure is operated to the changeover piston 20 to move it in the predetermined turning range allowing the communication, and the oil pressure is not operated to the changeover piston 20 in the other turning range breaking the communication.