To solve the problem that a U-shaped receiving groove for a roll pin make the roll pin easy to slip off upon assembling of a motor/pump, resulting in a difficulty in the motor/pump assembling and an occurring of toppling force working on the roll pin after the assembling, which shrinks the product life cycle of the pump/motor.

On the inner circumference 2f of the shaft hole of a cylinder block, a partially cylindrical-shaped receiving hole 2c is formed, the less than half of which circumference being opened inside. When the roll pin 10 is fitted in the receiving hole 2c, the slipping off of the pin is prevented in assembling, and the occurring of the toppling force is also prevented.