To provide a hydraulic power transmission capable of reducing the cost by simplifying a manufacturing process.

A cylindrical cover member 19 configurating a casing C and a left first side plate 30L for closing an opening part at a left end of the cover member 19 are connected by holding a tapered bottom part 19b formed on the left end of the cover member 19 between a flange part 30f formed on the left first side plate 30L and a ring gear 12 mounted on the flange part 30f. Further the oil sealing between the cover member 19 and the left first side plate 30L is performed by an O-ring 23 held in an annular space having a triangular cross section surrounded by the bottom part 19b, an outer peripheral face of the left first side plate 30L and the flange part 30f.