After a controller #1 has started giving an interpolating instruction, the length on an interpolation line is calculated by a first calculation means in synchronism with a clock signal from a clock synchronous circuit 45, and after having executed a first step for generating a synthetic locus-use frame 100 based upon the calculated value, the synthetic locus-use frame 100 is transmitted to controller #2 so that, after controller #2 has executed a second step for receiving a synthetic locus calculation-use frame from a receiving means, controllers #1, #2 execute a third step for calculating the position on the interpolation line based upon the synthetic locus-use frame 100 by using a second calculation means.