When detaching or attaching a rotary cylinder in a rotary cylinder device having a hydrodynamic bearing, a gap is formed between the end surface of a shaft and a thrust plate and this gap is maintained by inserting an insertion member between opposing surfaces of stronger parts of the device. In this state, the rotary cylinder is removed from or assembled to the device. Accordingly, the force required to remove or fix the cylinder from or to the device acts on the stronger parts of the device and is thus not transmitted to the thrust plate and end surface of the shaft.