[Problem] To facilitate mounting of a nozzle.[Solving Means] In a nozzle mounting structure for mounting a nozzle 20 penetrating through a reactor vessel 10 having a hemispherical concave inner surface, the nozzle mounting structure includes a removed concave portion 17 in which a base material 12 on an inner surface side of the reactor vessel 10 is removed in a symmetrical shape around a normal line N on the hemispherical concave inner surface of the reactor vessel 10, a flange 25 provided on the nozzle 20, formed in a same symmetrical shape as that of the removed concave portion 17 around the normal line N, and inserted into the removed concave portion 17, and a weld part 18 provided around the normal line N for welding the flange 25 to the reactor vessel 10.