PURPOSE: To calculate gear deflection by storing minimum and maximum values over the section of positions even if a timing pulse does not coincide with the position of the minimum value of deflection values.

CONSTITUTION: If pulses P1WP5 are received during the rise of deflection values on the way of a cycle, the minimum value before the increment and a temporary maximum value are stored. In the section between positions Q1 and Q2, the minimum value is M1 and the temporary maximum value is M2. In the section between the succeeding positions Q2 and Q3, the maximum value M3 of the rising cycle and the succeeding minimum value are stored. The maximum value Mmax and the minimum value Mmin in successively stored minimum values M1W Mn are found out and the gear vibration R is calculated from the formula. When the amplitude of the deflection value is rising at the generation of the pulse P2 even if the phase of the pulses P1WP5 does not coincide with that of the minimum value, the minimum value M1 before rising, the maximum value M3 continuously generated in the succeeding section and the minimum value M4 generated successively to the maximum value M3 are found and the gear deflection R is found out from the formula on the basis of respective minimum values.