A piston assembling apparatus includes a piston holding means including a guide tube portion which contains the piston and guides the piston into the cylinder bore. A guide tube support portion supports the guide tube portion to be displaceable in a radial direction thereof. A moving means moves at least one of the piston holding means and the cylinder block so as to locate them at an insertion preparation position where the guide tube portion and the cylinder bore are coaxial with each other. A pressing means presses the piston contained in the guide tube portion toward the cylinder block to insert the piston into the cylinder bore in a state in which the piston holding means and the cylinder block are located at the insertion preparation position, and the guide tube portion is supported by the guide tube support portion to be displaceable in the radial direction thereof.