A light emitting sensor emits light toward sheets carried along a carriage path and the quantity of light transmitted through each to the sheets is detected by a light receiving sensor. A predetermined sampling number of electric signals indicative of light quantity detected by the light receiving sensor is stored in a memory. A histogram of the light quantities stored in the memory is created. A light quantity corresponding to a maximum frequency for an underlying portion of the sheets is detected based on the created histogram. A multiple feed of the sheets is detected based on a variation in the light quantity of the maximum frequency for the underlying portion.