A power tool comprising: a reservoir reserving oil, and a cap attached to an opening of the reservoir, wherein the cap comprises: a cap body comprising a communicating path communicating an inside of the reservoir to an outside of the reservoir; and a valve member attached to the cap body and covering the communicating path, and the valve member, being a sheet-shaped portion and made from elastomer, comprises: an elastic part having a central portion that is capable of elastically deforming to both sides of an inner surface side and an outer surface side relative to a peripheral portion of the elastic part, wherein the inner surface receives a pressure inside the reservoir and the outer surface receives a pressure outside the reservoir; and a projecting part projecting from the elastic part, the projecting part having a slit on a top portion of the projecting part, characterized in that the elastic part and the projecting part are configured to: deform toward the inner surface side such that the slit of the projecting part opens when the pressure outside the reservoir becomes higher than the pressure inside the reservoir and a difference between the pressure inside the reservoir and the pressure outside the reservoir exceeds a predetermined value, and deform toward the outer surface side such that the slit of the projecting part opens when the pressure inside the reservoir becomes higher than the pressure outside the reservoir. a reservoir reserving oil, and a cap attached to an opening of the reservoir, wherein a cap body comprising a communicating path communicating an inside of the reservoir to an outside of the reservoir; and a valve member attached to the cap body and covering the communicating path, and the valve member, being a sheet-shaped portion and made from elastomer, comprises: an elastic part having a central portion that is capable of elastically deforming to both sides of an inner surface side and an outer surface side relative to a peripheral portion of the elastic part, wherein the inner surface receives a pressure inside the reservoir and the outer surface receives a pressure outside the reservoir; and a projecting part projecting from the elastic part, the projecting part having a slit on a top portion of the projecting part, characterized in that the elastic part and the projecting part are configured to: deform toward the inner surface side such that the slit of the projecting part opens when the pressure outside the reservoir becomes higher than the pressure inside the reservoir and a difference between the pressure inside the reservoir and the pressure outside the reservoir exceeds a predetermined value, and deform toward the outer surface side such that the slit of the projecting part opens when the pressure inside the reservoir becomes higher than the pressure outside the reservoir. an elastic part having a central portion that is capable of elastically deforming to both sides of an inner surface side and an outer surface side relative to a peripheral portion of the elastic part, wherein the inner surface receives a pressure inside the reservoir and the outer surface receives a pressure outside the reservoir; and a projecting part projecting from the elastic part, the projecting part having a slit on a top portion of the projecting part, deform toward the inner surface side such that the slit of the projecting part opens when the pressure outside the reservoir becomes higher than the pressure inside the reservoir and a difference between the pressure inside the reservoir and the pressure outside the reservoir exceeds a predetermined value, and deform toward the outer surface side such that the slit of the projecting part opens when the pressure inside the reservoir becomes higher than the pressure outside the reservoir.