A valve system for an internal combustion engine in which accumulating means can be provided without deteriorating the ease of assembly is provided. A variable valve control mechanism provides control to selectively open and close intake valves and exhaust valves. Pressurized oil supplied to the variable valve control mechanism is accumulated in an accumulator. The body of the accumulator is inserted into and fixed in a hole of a cylinder head. The length of a gap formed between the upper part of the body of the accumulator and a flat plate is set to be shorter than the length of a screw part of the body of the accumulator. Therefore, the accumulator can be replaced and maintained easily, and can be prevented from falling off.