A signal processing device includes a comparison unit to compare a signal level of an analog signal with a signal level of a reference signal; a selection unit configured to select the reference signal to be supplied to the comparison unit; and a switching unit capable of switching a signal line connected to an input terminal of the comparison unit such that a signal line via which the selected reference signal is transmitted is connected to the input terminal of the comparison unit, wherein the comparison unit includes a floating node as the input terminal, the selection unit includes a signal line in which a parasitic capacitance is caused between the signal line and the floating node as the input terminal of the comparison unit, and the signal line of the selection unit is configured to transmit an identical level of signal in multiple comparison processes of the comparison unit.