To tighten or loosen an object to/from a rotary base by axially moving the object without rotating a bolt.

When the object such as a gear is mounted on the rotary base by the bolt, a nut and a washer by using a single power tool having a housing part and a driving part mounted on the same shaft in a state of receiving the equal power in the directions opposite to each other, a leg part of the bolt inserted into an central opening part of the object is screwed on the rotating shaft of the rotary base, a first part including a sleeve-washer assembly composed of a sleeve screwed with a head part of the bolt and the washer rotated integrally with the sleeve is engaged with the head part of the bolt in a state of bringing the washer into contact with the object, and one of the housing part and the driving part of the power tool is engaged with the first part. The other of the housing part of and the driving part of the power tool is engaged with a second part including a nut engaged with the sleeve of the first part, whereby the object is tightened or loosened to/from the rotary base by being axially moved without rotating the bolt, by rotating one of the first part and the second part while keeping the other in a non-rotated state by the power tool.