To improve estimating accuracy of a spindle position increment value by providing a means storing a spindle position command calculated from a stored spindle position command increment value, means storing a spindle position calculated from a stored spindle position increment value, and a means determining an estimated value of the spindle position increment value.

An arithmetic device in a command forming device is provided with a means which arithmetically stores a spindle position command rs(i+M-1), rs(i+M-2),..., rs(i-K-Nn) from a spindle position command increment value Δrs stored in a memory. From a spindle position increment value Δys stored in the memory, a means is provided, which stores a spindle position ys(i-K), ys(i-K-1),..., ys(i-K-Na). Further, from the stored spindle position command rs and the spindle position ys, a means is provided, which determines as a formula an estimated value of the spindle position increment value Δys(i+m), m=D, D+1, ...D+M.