To decide the shape of a coil capable of preventing the generation of a kink phenomenon at arising part of the coil embedded in a fusion part of an electrofusion joint.

This method of deciding the shape of a coil has a fragment extracting step for extracting a simulation range (step S1), an input value specifying step for specifying a value necessary to the simulation (step S2), a displacement computing step for computing the displacement of each wire fragment in the spiral axial direction obtained as a result of the deformation of a wire by the heat generated when a voltage is applied to the wire with a geometric nonlinear analysis related to a large deformation of the wire fragment in the extracted range by using the input value specified in the step S2 (step S3), and a comparing step for comparing the described deformation with a distance between a wire adjacent to the rising part with one pitch and each wire fragment per each wire fragment (step S4).