Abstract: In a self-propelled walking mower having a deck as a machine frame for supporting an engine, mowing cutter and front wheels, a transaxle-type transmission for driving rear wheels is housed at a rear end portion of the deck in a transmission casing an upper half of which is provided by a portion of top wall of the deck and by a peripheral wall which is formed integrally with the deck and projected downwardly from the top wall of deck. The transmission may be assembled in a lower half of the transmission casing, or alternatively in the upper half, and then the two casing halves may be secured together for completing an essential part of the mower drive system. The upper casing half integral with the deck contributes not only to an easy assemblage of the drive system but also to a reduction of weight and size of the mower.