PURPOSE: To enhance the reliability of a cross-flow type turbine runner in mechanical strength, by casting discs between a prefabricated rotary shaft and a predetermined number of blades to integrally form the runner.

CONSTITUTION: There are provided a mold 13 which is horizontally split into four sections, and disc forming molds 11 are arranged having the split surfaces 12 of the four sections as their upper surfaces. Further, a prefabricated rotary shaft 1 adapted to be piercing through the disc forming molds 11, and a predetermined number of blades 3 are positioned in the mold 13. Further, the rotary shaft 1 and the blades in parts within the disc forming molds 11 are formed therein with through-holes 1a, 3a. Molten metal is fed into the disc-like forming molds 11 from a gate 15 to produce a cross-flow water turbine runner.