In a state where a sheet exists, an current I is gradually increased. When an voltage V reaches a light receiving level Vx, the current I at the time is stored as I1. The voltage V immediately before the voltage V reaches the light receiving level Vx is stored as V1. In a state where no sheet exists, the current I is gradually decreased from the above I1. When the voltage V reaches a non-light receiving level Vy, the current I at the time is stored as I2. The voltage V immediately before the voltage V reaches the non-light receiving level Vy is stored as V2. Then, the current I at the time of normal operation is set within a range between I1 and I2. A reference voltage Vs is set within a range between V1 and V2.