A transmission device (1) of a walking type self-traveling lawn mower (4) in which the reliable transmission of a rotating power is achieved as in the case of a shaft drive system by solving the problem of the durability of the transmission belt of conventional transmission devices while basically adopting the transmission device of a belt drive system. The transmission device (1) comprises a chain drive mechanism (2) and a rotating direction changing mechanism (3). The chain drive mechanism (2) is constituted by winding a transmission chain (23) around a drive sprocket (21) fitted to an output shaft (411) and rotating horizontally and a driven sprocket (22) connected to the rotating direction changing mechanism (3) and rotating horizontally. The rotating direction changing mechanism (3) comprises a drive bevel gear (31) coaxial with the driven sprocket (22) rotating horizontally, a driven bevel gear (32) meshed with the drive bevel gear (31) rotating vertically, and an auxiliary output shaft (33) coaxial with the driven bevel gear (32) rotating vertically.
