A nozzle box for being arranged in an axial flow turbine having a turbine rotor and a first-stage nozzle configured to eject a working fluid for the turbine rotor, wherein the nozzle box forms a plurality of working fluid channels for leading the working fluid to the first-stage nozzle of the axial flow turbine, each working fluid channel having a channel center line and each working fluid channel comprising a pair of pipes comprising: a lead-in pipe into which the working fluid is led, the lead-in pipe extending in a direction perpendicular to a horizontal plane, the horizontal plane for being arranged along the center axis of the turbine rotor; and a bent pipe connected to the lead-in pipe and formed so as to change the direction of the channel center line to a predetermined angle to the horizontal plane or to be parallel to the horizontal plane; wherein each working fluid channel further comprises a segment of an annular pipe, wherein the annular pipe is for covering the turbine rotor from an outer peripheral side of the turbine rotor, wherein in the circumferential direction the annular pipe is divided into a first part and a second part, each part being connected with at least two pairs of pipes, wherein for each pair of pipes the annular pipe segment forms an outlet of the working fluid channel for leading the working fluid of a pair of pipes to the first-stage nozzle while spreading the working fluid in a circumferential direction of the turbine rotor, wherein from an inlet of the lead-in pipe toward an outlet of the annular pipe, a channel width of the working fluid channel in a first direction (Sa-1 to Sn-1) intersecting with the channel center line gradually increases and a channel width of the working fluid channel in a second direction (Sa-2 to Sn-2) intersecting with the channel center line and perpendicular to the first direction gradually decreases, wherein the channel width in the first direction and the channel width in the second direction lie on the same channel cross section perpendicularly intersecting with the channel center line of the working fluid channel, and when the channel width in the first direction and the channel width in the second direction are different from each other, the channel cross section has a longitudinal direction, wherein the channel width in the first direction is in the longitudinal direction of the channel cross section so that the steam channel width in the first direction is the largest channel width of this channel cross section, and characterized in that the area of the channel cross section monotonously changes from the inlet of the lead-in pipe toward the outlet of the annular pipe. a lead-in pipe into which the working fluid is led, the lead-in pipe extending in a direction perpendicular to a horizontal plane, the horizontal plane for being arranged along the center axis of the turbine rotor; and a bent pipe connected to the lead-in pipe and formed so as to change the direction of the channel center line to a predetermined angle to the horizontal plane or to be parallel to the horizontal plane; wherein each working fluid channel further comprises a segment of an annular pipe, wherein the annular pipe is for covering the turbine rotor from an outer peripheral side of the turbine rotor, wherein in the circumferential direction the annular pipe is divided into a first part and a second part, each part being connected with at least two pairs of pipes, wherein for each pair of pipes the annular pipe segment forms an outlet of the working fluid channel for leading the working fluid of a pair of pipes to the first-stage nozzle while spreading the working fluid in a circumferential direction of the turbine rotor, wherein from an inlet of the lead-in pipe toward an outlet of the annular pipe, a channel width of the working fluid channel in a first direction (Sa-1 to Sn-1) intersecting with the channel center line gradually increases and a channel width of the working fluid channel in a second direction (Sa-2 to Sn-2) intersecting with the channel center line and perpendicular to the first direction gradually decreases, wherein the channel width in the first direction and the channel width in the second direction lie on the same channel cross section perpendicularly intersecting with the channel center line of the working fluid channel, and when the channel width in the first direction and the channel width in the second direction are different from each other, the channel cross section has a longitudinal direction, wherein the channel width in the first direction is in the longitudinal direction of the channel cross section so that the steam channel width in the first direction is the largest channel width of this channel cross section, and characterized in that the area of the channel cross section monotonously changes from the inlet of the lead-in pipe toward the outlet of the annular pipe.