In an optical sensor, a detecting part includes detecting elements, and a light-shielding portion providing directional characteristics of light to the detecting elements such that the detecting elements indicate different intensities of light received in a specified direction. A signal processing part has an adjustment value for processing detection signals. The adjustment value is set as follows. Based on information of an inclination angle of a window glass of a vehicle to which the detecting part is attached, from the detecting elements, a detecting element having a directional characteristic of light suitable for the inclination angle is selected. The adjustment value is then set so as to coincide a signal value of a detection signal of the selected detecting element with a target value, when the detecting part is irradiated with light in a predetermined direction in a state where the detecting part is inclined at the inclination angle.