next at step s 5 it is determined whether or not secondary evaluation values corresponding to secondary reference characteristic values or diagnostic characteristic values should be calculated in case of yes the secondary evaluation values are calculated at step s 6 and the calculated secondary evaluation values are recorded at step s 7 in case of no the processing proceeds to step s 8 here the secondary evaluation values are calculated as shown in fig8 thus based on the evaluation values calculated at step s 3 for various velocities maximum value minimum value average value and deviation are calculated as an example if in one measurement for movement patterns of v 1 v 2 v 3 the maximum values of estimated external torque disturbance for respective velocities t max1 t max2 t max3 are measured with the relation of t max1 lt t max2 lt t max3 and if maximum value of the primary evaluation values is to be used as a secondary evaluation value t max3 is the secondary evaluation value if the average value of the primary evaluation value is to be used as a secondary evaluation value the secondary evaluation value is t max1 t max2 t max3 3 by calculating the secondary evaluation value in this way measurement data can be evaluated comprehensively next at step s 8 the evaluation value calculated from the estimated torque obtained when electrode tip 14 a moves normally and work piece 11 is pressurized by a specified pressurizing force is compared with the diagnostic evaluation value calculated from the estimated torque obtained when the operational state of the movable side electrode.