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 (103a, 103T) that receives the expanded light, the light receiving part (103a, 103T) being a heat-absorbing plate; a first distribution part disposed adjacent to the light receiving part (103a, 103T) and that introduces or leads out a medium from a first opening and distributes the medium to absorb heat generated by the light in the light receiving part (103a, 103T), 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 from a second opening and distributes the medium 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 (103b, 130c); wherein the communicating part is formed in the center of the separation plate; the light receiving part (103a, 103T) and the separation plate are each tilted with respect to a horizontal direction; and the medium 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 (103a, 103T) that receives the expanded light, the light receiving part (103a, 103T) being a heat-absorbing plate; a first distribution part disposed adjacent to the light receiving part (103a, 103T) and that introduces or leads out a medium from a first opening and distributes the medium to absorb heat generated by the light in the light receiving part (103a, 103T), 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 from a second opening and distributes the medium 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 (103b, 130c); a light receiving part (103a, 103T) that receives the expanded light, the light receiving part (103a, 103T) being a heat-absorbing plate; a first distribution part disposed adjacent to the light receiving part (103a, 103T) and that introduces or leads out a medium from a first opening and distributes the medium to absorb heat generated by the light in the light receiving part (103a, 103T), 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 from a second opening and distributes the medium 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 (103b, 130c); wherein the communicating part is formed in the center of the separation plate; the light receiving part (103a, 103T) and the separation plate are each tilted with respect to a horizontal direction; and the medium 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.