A chain saw comprising: a chain saw body; a linear motion mechanism disposed on the chain saw body; a guide bar attached to the chain saw body and being capable of moving reciprocally relative to the chain saw body; and a saw chain provided with tension between the chain saw body and the guide bar, wherein the linear motion mechanism comprises: a rotating shaft comprising an external thread portion at least at a part of an area in a length direction; and a linear motion member engaging with the guide bar and comprising an opening (60, 100) through which the external thread portion of the rotating shaft passes, characterized in that the opening (60, 100) of the linear motion member has a cross-sectional shape that enables the linear motion member to move between a first position and a second position along a radial direction of the rotating shaft, an inner thread is formed on an inner surface of the opening (60, 100), the inner thread engages with the external thread portion of the rotating shaft when the linear motion member moves to the first position, and the engagement between the inner thread and the external thread portion is released when the linear motion member moves to the second position, the linear motion member comprises a retaining unit (56, 58) capable of retaining the linear motion member at the first position, and the retaining unit (56, 58) comprises an elastic member biasing the linear motion member toward the first position. a chain saw body; a linear motion mechanism disposed on the chain saw body; a guide bar attached to the chain saw body and being capable of moving reciprocally relative to the chain saw body; and a saw chain provided with tension between the chain saw body and the guide bar, wherein the linear motion mechanism comprises: a rotating shaft comprising an external thread portion at least at a part of an area in a length direction; and a linear motion member engaging with the guide bar and comprising an opening (60, 100) through which the external thread portion of the rotating shaft passes, characterized in that the opening (60, 100) of the linear motion member has a cross-sectional shape that enables the linear motion member to move between a first position and a second position along a radial direction of the rotating shaft, an inner thread is formed on an inner surface of the opening (60, 100), the inner thread engages with the external thread portion of the rotating shaft when the linear motion member moves to the first position, and the engagement between the inner thread and the external thread portion is released when the linear motion member moves to the second position, the linear motion member comprises a retaining unit (56, 58) capable of retaining the linear motion member at the first position, and the retaining unit (56, 58) comprises an elastic member biasing the linear motion member toward the first position. a rotating shaft comprising an external thread portion at least at a part of an area in a length direction; and a linear motion member engaging with the guide bar and comprising an opening (60, 100) through which the external thread portion of the rotating shaft passes, characterized in that the opening (60, 100) of the linear motion member has a cross-sectional shape that enables the linear motion member to move between a first position and a second position along a radial direction of the rotating shaft, an inner thread is formed on an inner surface of the opening (60, 100), the inner thread engages with the external thread portion of the rotating shaft when the linear motion member moves to the first position, and the engagement between the inner thread and the external thread portion is released when the linear motion member moves to the second position, the linear motion member comprises a retaining unit (56, 58) capable of retaining the linear motion member at the first position, and the retaining unit (56, 58) comprises an elastic member biasing the linear motion member toward the first position.