PURPOSE: To measure a wiping rate with a high precision by inputting coordinate values of the outline of a windshield and the outline of the wiping range of a windshield wiper to a position coordinate input device along these outlines and calculating the area of the wiping range and a wiping ratio on a basis of these inputted data.

CONSTITUTION: A plan 3 where a windshield 1 and a wiping range 2 of the windshield wiper are indicated is fixed onto a writing surface 4 of a graphic tablet. Magnetostrictive wires are stretched in X and Y directions into a lattice shape under the writing surface, and electromagnetic waves are moved along this lattice. When a cursor 5 provided with a pickup coil at the tip is moved, a change of the magnetic field due to moving electromagnetic waves of the writing surface 4 is taken out by the pickup coil and is converted to coordinate data and is sent to an electronic computer 6. On a basis of this coordinate data, areas of the front window glass and the wiping range and a ratio of the area of the latter to the area of the former are calculated by the electronic computer.