A rotary cutter for a mower capable of limiting the reeling out of a cord, when the free end of the cord is worn out to only when the rotary cutter is rotated at low speed thereby preventing pebbles or the like from being scattered. The rotary cutter is disengaged without the component parts being separated thereby facilitating reassembly. The rotary cutter includes a cord for mowing grass on being rotated, a reel for containing the cord and reeling out the cord intermittently while being rotated, a cover covering a case containing the reel, and a plunger projected downward from the case and movable in an axial direction by being pressed for intermittently rotating the reel. A ratchet is held at a constant position by an urging element which is moved outward to open against the direction of the urging element by the centrifugal force produced by rotating the rotary cutter at a high speed and is engaged with the plunger to thereby prevent the plunger from moving in the axial direction. The urging element includes a return spring urging the plunger in an outer direction. The reel and the plunger are separate members contained in the case and cover and are detachably connected to each other, the reel being intermittently rotated by moving the plunger in the axial direction and the case and the reel separating from the plunger when the case and the cover are detached from each other.