A hydraulic transmission apparatus comprises an input unit to which power from a power source is applied in a first direction, a direction changing unit hydraulically coupled to the input unit for extracting power in a second direction which is different from the first direction, and a case supporting the input unit and the direction changing unit. The input unit and the direction changing unit comprise an oil pump and an oil motor mounted respectively on one surface and another surface of the case. The case is filled with working oil for operating the oil pump and the oil motor. An operating device for a lawn mower has a first power transmitting path for transmitting power from the power source to a drive wheel, the first power transmitting path having a hydraulic transmission apparatus and a first clutch for selectively rotating and stopping the drive wheel, and a second power transmitting path for transmitting power from the power source to a grass cutter, the second power transmitting path including a second clutch for selectively rotating and stopping the grass cutter. The first clutch is independently operable. The second clutch has a mechanism for inactivating the first clutch in unison therewith when the second clutch is inactivated.