A differential circuit and an amplifier circuit for reducing an amplitude difference deviation, performing a full-range drive, and consuming less power are disclosed. The circuit includes a first pair of p-type transistors and a second pair of n-type transistors. A first current source and a first switch are connected in parallel between the sources of the first pair of transistors, which are tied together, and a power supply VDD. A second current source and a second switch are connected in parallel between the sources of the second pair of transistors, which are tied together, and a power supply VSS. The circuit further includes connection changeover means that performs the changeover of first and second pairs between a differential pair that receives differential input voltages and a current mirror pair that is the load of the differential pair. When one of the two pairs is the differential pair, the other is the current mirror pair. In a differential amplifier circuit, there is provided an added transistor connected in parallel to a transistor, which is one transistor of a differential pair transistors, whose control terminal is a non-inverting input terminal. The added transistor has a control terminal for receiving a control voltage which is set so that, when an input voltage applied to the non-inverting input terminal is in a range in which the transistor whose control terminal is the non-inverting input terminal is turned off, the added transistor is turned on.