It is intended to provide a power generating apparatus of renewable energy type which can transmit large torque and has a low-cost and readily processable structure of fixing a hydraulic pump to the main shaft and a method of mounting the hydraulic pump. A plurality of key slots 25 are formed at an end on the back-end side of the main shaft 4. A plurality of keys 30A, 30B are inserted in the key slots 25. The keys 30A, 30B include a reference key 30A to be inserted in one of the key slots and a split key 30B to be inserted in the other of the key slots. The split key 30B is separated into a first key piece 34 having a projection to be inserted in a first groove formed in the main shaft and a second key piece 36 having a depression to be inserted in a second groove formed in the cylindrical member. The pair of key pieces 34, 36 are arranged so that the projection and the depression engage with each other. The projection and the depression of the first and second key pieces have side surfaces 34A, 34B, 36A and 36B formed along a radial direction of the main shaft 4. The side surfaces 34A, 34B, 36A and 36B are contact surfaces of the first and second key pieces 36 and incline with respect to the axial direction of the main shaft.