To provide lock structure of a rotary valve capable of surely locking a handle with simple operation without using a separate member.

This lock structure of a rotary valve comprises: a valve element 8 built in a valve body 1; a stopper 19 provided outside the valve body 1; a handle 20 for opening/closing the valve element 8 and regulated in the turning amount thereof by the stopper 19; a lock member 26 turnably fitted to the handle 20 and formed with an engagement recessed part 29 to be engaged with the outer surface of the stopper 19 in operation for full opening or full closing the valve element 8; and a spring for energizing the lock member 26 in a direction for engagement with the stopper 19.