An image reading device includes a reading unit and a controller connected to the reading unit through a transmission path. The controller includes: a first clock generator which generates a first reference clock; a second clock generator which generates a second reference clock; a line synchronizer which generates a line synchronization signal based on the first reference clock; a clock transmitter which transmits the line synchronization signal and a unit clock based on the first reference clock to the reading unit; and a processor which execute a process based on the second reference clock and which is synchronized with the line synchronization signal. The reading unit includes a reading sensor which reads a document line by line based on the unit clock and the line synchronization signal.