In a chain saw, an operating shaft member axially rotated by a manual operation of a rotating lever is disposed so that an axial direction thereof may be inclined against an attaching surface of a guide bar in which a saw chain is wound around a circumference thereof, a driving bevel gear attached to an end portion of the operating shaft member meshes with a driven bevel gear attached to an end portion of a rotating shaft member, and the rotating shaft member is axially rotated. Thus, the guide bar is moved in a longitudinal direction thereof to adjust tension of the saw chain.