A nozzle box 10 includes: a lead-in pipe 20; a bent pipe 30 connected to the lead-in pipe 20 and formed so as to change a direction of a channel center line 50 to an axial direction of a turbine rotor 212; and an annular pipe 40 connected to the bent pipe 30 and leading steam to a first-stage nozzle 213a while spreading the steam in a circumferential direction of the turbine rotor 212. In the steam channel lead-in part structure 10, from an inlet of the lead-in pipe 20 toward an outlet of the annular pipe 40, steam channel widths Sa-1 to Sn-1 in a first direction intersecting with the channel center line 50 gradually increases and steam channel widths Sa-2 to Sn-2 in a second direction intersecting with the channel center line 50 and perpendicular to the first direction gradually decreases.