PURPOSE: To always carry out automatic adjustment appropriately without variation in light quantity of a light emitting element.

CONSTITUTION: An adjustment range with standard toner concentration is provided on a transfer belt of a copying machine, etc., where the toner concentration sensor is adapted, and at the same time, an arithmetic means calculating the light quantity of the light emitting element where the light receiving element is made to be a specified output and an adjustment means adjusting the light quantity of the light emitting element to be the value calculated by the arithmetic means are provided on the exterior of the toner sensor main body.