To automatically assemble a shaft assembly of a swash plate type fluid machinery regardless of whether or not a center hole is present on an end surface of a piston.

The assembly assembling device A1 assembles a shaft assembly to which a plurality of pistons 2 are attached at an outer peripheral portion of a swash plate 3b via a shoe 4. The assembling device A1 includes: a shaft support 41 for supporting a shaft having the swash plate 3b in a vertical axis posture; a guide member 20; a shoe incorporation mechanism 30; a carrier 10 for conveying the plurality of pistons 2; and a piston holding part 41 for holding the plurality of pistons 2 guided to the outer peripheral portion of the swash plate 3b at positions equally distributed in the circumferential direction of the swash plate. The guide member 20 includes a first guide surface and a second guide surface 23, guides each of the pistons 2 to the outer peripheral portion of the swash plate 3b while holding one of a pair of the shoes 4 incorporated in the neck of the piston 2 in slidable contact with a first guide surface and the other of the shoes in slidable contact with the second guide surface 23, and aligns the pair of the shoes 4 with both end surfaces of the swash plate 3b.