An optical attenuator comprising: a branch member that splits light to be measured; an expansion member that expands a shape of the split light; and an absorption member that absorbs energy of the expanded light, characterized in that the absorption member comprises: a light receiving part that receives the expanded light, the light receiving part being a heat-absorbing plate ; a first distribution part disposed adjacent to the light receiving part and that introduces or leads out a medium (W) from a first opening and distributes the medium (W) to absorb heat generated by the light in the light receiving part , the first opening being formed on an outer edge of the first distribution part , a second distribution part that leads out or introduces the medium (W) from a second opening and distributes the medium (W) through a communicating part that communicates with the first distribution part , the second opening being formed on an outer edge of the second distribution part ; and a separation plate separating the first and second distribution parts ; wherein the communicating part is formed in the center of the separation plate ; the light receiving part and the separation plate are each tilted with respect to a horizontal direction; and the medium (W) is introduced from the first opening and led out from the second opening , or introduced from the second opening and led out from the first opening . a branch member that splits light to be measured; an expansion member that expands a shape of the split light; and an absorption member that absorbs energy of the expanded light, characterized in that the absorption member comprises: a light receiving part that receives the expanded light, the light receiving part being a heat-absorbing plate ; a first distribution part disposed adjacent to the light receiving part and that introduces or leads out a medium (W) from a first opening and distributes the medium (W) to absorb heat generated by the light in the light receiving part , the first opening being formed on an outer edge of the first distribution part , a second distribution part that leads out or introduces the medium (W) from a second opening and distributes the medium (W) through a communicating part that communicates with the first distribution part , the second opening being formed on an outer edge of the second distribution part ; and a separation plate separating the first and second distribution parts ; a light receiving part that receives the expanded light, the light receiving part being a heat-absorbing plate ; a first distribution part disposed adjacent to the light receiving part and that introduces or leads out a medium (W) from a first opening and distributes the medium (W) to absorb heat generated by the light in the light receiving part , the first opening being formed on an outer edge of the first distribution part , a second distribution part that leads out or introduces the medium (W) from a second opening and distributes the medium (W) through a communicating part that communicates with the first distribution part , the second opening being formed on an outer edge of the second distribution part ; and a separation plate separating the first and second distribution parts ; wherein the communicating part is formed in the center of the separation plate ; the light receiving part and the separation plate are each tilted with respect to a horizontal direction; and the medium (W) is introduced from the first opening and led out from the second opening , or introduced from the second opening and led out from the first opening . <EOS>