In a scanner, a pulse generator generates a pulse signal, a transmissive optical sensor includes a light projector that projects pulsed light corresponding to the pulse signal and a light receiver disposed so as to face the light projector, a peak hold circuit holds a peak value of a voltage value indicating a light reception amount of the pulsed light in the light receiver, and a first current supply circuit increases a light projection amount of the pulsed light projected from the light projector by increasing a current supplied to the light projector when the peak value is smaller than a reference value and decreases the light projection amount of the pulsed light projected from the light projector by decreasing the current supplied to the light projector when the peak value is larger than the reference value.