To prevent an oil temperature rise in a hydraulic fluid circulating in an HST circuit in an axle driving device having an HST.

This axle driving device is composed of a housing for forming an oil reservoir, the constitution being an axle driving HST 8 arranged in the housing and having a hydraulic pump 11, a hydraulic motor 21 and first/ second circuits 93, 94 for fluidly connecting both and the constitution arranged by communicating with at least either one of a charge oil passage 82 being respectively arranged in the first/second circuits 93, 94 and flowing oil in the direction of the circuits 93, 94 from the oil reservoir and the first/second circuits 93, 94 being a drain oil passage 101 having an orifice 116 and being arranged in a position separate from the charge circuit 82 for flowing oil in the direction of the oil reservoir from the circuits 93, 94.