a lawn mower comprises an operation arm , a driven shaft rotatably supported inside the operation arm and having two opposite ends , a cutter operatively connected to one end of the driven shaft , a power source operatively connected to the other end of the driven shaft for generating a drive power to be transmitted to the cutter through the driven shaft , wherein said power source is selective from an engine and an electric motor , an arm side joint case provided on the end of the operation arm , a power source side joint case provided on a housing of the power source and detachably and selectively connected to the arm side joint case , and a centrifugal clutch incorporated in the arm side joint case and having a first member connected to the driven shaft , a second member detachably and selectively connected to the power source and a third member provided to engage the first member and the second member according to the centrifugal force applied thereto .