A riser position setting method is provided to set a riser by detecting a thick part by grouping cells with the same depth in the same group and thinning the groups in order. The riser position setting method for successively removing the outside of an object in the predetermined thickness by a computer simulation and then selecting the final portion as the position of a riser comprises a cell splitting step of dividing the shape of the object into cells of the same size by the computer simulation(S20); a first grouping step of grouping the cells formed in the same depth from the outside, into the same group(S21); a first thinning step of removing the groups from the outside in order(S22); and a riser setting step of setting the position of the final group as the position of the riser(S24).
