A lawn mower comprising a cutting unit disposed between front and rear wheels, a grass discharge duct extending rearwardly from the cutting unit, and a grass collecting container defining a top opening for receiving grass clippings sent through the discharge duct. The grass collecting container includes a mouthpiece extending along peripheries of the top opening to rest on a U-shaped support frame mounted at a rear end of the mower. The mouthpiece is connected to the support frame by a projection and bore engagement. The support frame includes a holder defining an upwardly opening recess, and the container has a cover defining a downwardly opening recess at a position opposed to the holder. The two recesses define an opening for receiving a rear end of the grass discharge duct. The grass discharge duct includes an annular engagement portion extending peripherally of the rear end thereof for engagement with inner edges of the two recesses.