A reel lawn mower which has a connection structure for connecting a reel cutting unit to a main body. The reel cutting unit has a spiral cutting reel which is rotated by a prime mover to cut grass together with a bedknife. In the connection structure, in order to connect the reel cutting unit to the main body so that the reel cutting unit rolls around a virtual horizontal line perpendicular to the shaft center of the cutting reel in the center of the axial direction of the cutting reel, the reel cutting unit includes a connecting arm with an arc portion shaped so as to follow a virtual arc centered on the virtual horizontal line. The connecting arm is slidably supported so as to prevent the arc portion from coming off the virtual arc.