PURPOSE: To provide a device of a mechanism which is adjustable for valve timing during an operation for speed and/or load characteristic and is simple, in an adjustable rotary valve assembly for internal combustion engine.

CONSTITUTION: A rotary valve assembly 12 is provided with a rotary valve plate formed by a pair of valve leaves 38, 40 which are coaxially mounted in overlapping relation and constitute a circular arc opening valve port. The valve plate is rotatably mounted to extend across an engine valve passage such as an intake passage in a two-stroke engine. By this valve plate, the engine valve passage is opened and closed. An adjusting sleeve 50 is provided so as to rotate the valve leaves 38, 40 during an operation and the valve leaves 38, 40 are rotated so as to adjust valve timing according to one or two or more selected operating conditions.