This invention discloses a switching constant current source circuit including a first current path for supplying a constant current, a first MOS transistor, one end of a current path of which is connected to the first current path, the other end of the current path of which is connected to a second current path, and a gate of which is applied with a digital signal corresponding to a logical amplitude, a second MOS transistor, one end of a current path of which is connected to the first current path, the other end of the current path of which is connected to a third current path, and which performs a switching operation complementary with the first MOS transistor, and level conversion means for fetching a change in voltage in the first current path caused by a change in current flowing through the first current path according to an operation of the first MOS transistor in response to the digital signal, and alternately applying a first level for disabling the second MOS transistor, which operates complementarily with the first MOS transistor, and a second level for enabling the second MOS transistor to output a predetermined current.