PURPOSE: To decide normal or failure with high accuracy and to reduce failure caused by any artificial error by preparing a straight line expression by the method of least squares while using a measured coordinate group inputted onto an input board by using a pen, calculating expected coordinates and comparing them with the measured coordinate group.

CONSTITUTION: An operator draws a diagonal on the input board of a pen point device 10 by using a stylus pen 9 just as tracing a diagonal 11 displayed while using a ruler 14. With this work, coordinate positions on the input board are detected and a diagnostic program can acquire all the coordinates on the input board detected by an antenna array. When the input of the measured coordinate group is detected, the diagnostic program prepares the expression of a straight line composed of inputted coordinates from all the measured X and Y coordinates by using the method of least square. Next, all the measured X and Y coordinates are substituted into this straight line expression, all the expected XYS coordinates are calculated, the respective coordinates are compared and when its difference is settled within the range of allowable error provided for the diagnostic program, normality is judged but when it gets out of the range, abnormality is judged and displayed on an LCD 8.