A grass collecting apparatus for a lawn mower comprises a frame (22) supported by a vehicle body, a container supporting member (26) for supporting a grass collecting container (21) for storing grass clippings and including a container supporting surface (27) for receiving an opening edge of the grass collecting container, and a container cover (23) mounted on the frame to be oscillatable about a horizontal axis between a closed position and an opened position. An engaging member (35) is mounted on the container cover (23). The engaging member (35) is engageable with the opening edge (21b) of the grass collecting container (21) with oscillation of the container cover (23) from the opened position to the closed position, thereby positioning and maintaining the grass collecting container (21) on the container supporting member (26), and disengageable from the opening edge (21b) of the grass collecting container (21) with oscillation of the container cover (23) from the closed position to the opened position.