Rotation speed calculation device

PROBLEM TO BE SOLVED: To obtain a rotational speed calculating device capable of reducing an interruption load on a CPU at a high speed rotation without imposing a new cost up to an independent rotary shaft.SOLUTION: Rotational speed operating means 31-33 includes an interruption processing section 71 that processes an interruption of a pulse input; a pulse input interruption processing permit/prohibit switch 72 that permits/prohibits the interruption processing in the interruption processing section 71; and a rotational speed operating section 68 that operates the rotational speed using the pulse input processed by the interruption processing section 71. The interruption processing section 71 includes a pulse count processing section 65 that counts the input pulses; a time measuring section 60 that measures the time of an input pulse; and a time storing processing section 70 that stores the time measured by the time measuring section 60. When the number of the pulses counted by the pulse count processing section 65 is a predetermined value or more in a predetermined time, the interruption processing by the interruption processing section 71 is prohibited.