To enable the production of an automobile with high operability and high safety by realizing a practical rain drop sensor having high reliability and durability for automating wipers for a window glass of the automobile.

In place of a direct method of electrically and two-dimensionally measuring the rain water left in a wiper wiping range and using this measurement value as an input value for control, a rain drop sensor capable of electrically and three-dimensionally measuring the mean residual rain water quantity corresponding to a value obtained by subtracting the scattered rain water quantity from the flow-in rain water quantity is provided, and a control method using this sensor output as an input value is used. A linear positive electrode and a linear negative electrode respectively having a low-height are arranged opposite to each other with a narrow space to hold the rain water corresponding to the mean quantity on a window surface in a groove between both the electrodes, and the electric three-dimensional measurement of the rain quantity on the window surface is enabled.