There is provided a signal processing apparatus including a variable capacitor and a switching portion for switching the circuit mode between a sampling mode, in which the variable capacitor samples an input signal, a holding mode, in which a charge gained by sampling the input signal is held in the variable capacitor, and an output mode for outputting the charge stored in the variable capacitor, wherein the variable capacitor is provided with an input terminal through which the input signal is inputted in the sampling mode, a control terminal to which a first control signal which decreases the capacitance of the variable capacitor to a value below the capacitance in the sampling mode is inputted in the output mode, and a second control signal having a predetermined reference voltage is inputted in the holding mode, where an insulating layer is provided between the control terminal and the input terminal.