A conventional rotary valve in which is provided with a locking mechanism for fixing the degree of aperture so as to prevent misoperation and reduce accidents caused thereby, and even a previously installed valve can be converted to a valve having a locking mechanism, simply by replacing the handle alone. In a rotary valve wherein a valve element that is engaged with a stem is turned by turning the stem, which is connected to a handle, so as to open and close the valve, and having a top flange that is provided on a neck of a valve body that supports the stem, a notch is formed at the peripheral edge of the top flange, and a locking member, having a protrusion that can be latched in the notch in the top flange by way of a torsion spring, is pivotably supported at the center of the handle or on a grip portion thereof.