A measuring apparatus comprising: a physical amount transformer configured to transform a physical amount into a current; a measuring circuit configured to measure an output current of the physical amount transformer; a power supply circuit configured to obtain, as an input into the power supply circuit, an output current of the measuring circuit; a circuit switching unit configured to switch between a first state and a second state, the first state constituting a circuit inputting the output current of the measuring circuit into the power supply circuit, the second state constituting a circuit directly inputting the output current of the measuring circuit into the physical amount transformer; a rectifier circuit configured to rectify the output current transformed by the physical amount transformer into a direct current, wherein the output current of the physical amount transformer is an alternating current; and a switching controller configured to be driven by the power supply circuit to control the circuit switching unit, wherein the switching controller is configured to control the circuit switching unit such that the circuit switching unit is in the first state during waiting which is a period during which at least a period of measurement is excluded, and the switching controller is configured to control the circuit switching unit such that the circuit switching unit is in the second state during measurement which is said period of measurement, wherein the physical amount transformer comprises a connection portion (21, 22) used to take out an alternating current, the measuring circuit comprises a detector configured to detect the current, a current input terminal of the detector is connected to one end of the connection portion (21, 22), the rectifier circuit is connected onto a current output terminal side of the detector, and the circuit switching unit is connected between the current output terminal side of the detector and the rectifier circuit. a physical amount transformer configured to transform a physical amount into a current; a measuring circuit configured to measure an output current of the physical amount transformer; a power supply circuit configured to obtain, as an input into the power supply circuit, an output current of the measuring circuit; a circuit switching unit configured to switch between a first state and a second state, the first state constituting a circuit inputting the output current of the measuring circuit into the power supply circuit, the second state constituting a circuit directly inputting the output current of the measuring circuit into the physical amount transformer; a rectifier circuit configured to rectify the output current transformed by the physical amount transformer into a direct current, wherein the output current of the physical amount transformer is an alternating current; and a switching controller configured to be driven by the power supply circuit to control the circuit switching unit, wherein the switching controller is configured to control the circuit switching unit such that the circuit switching unit is in the first state during waiting which is a period during which at least a period of measurement is excluded, and the switching controller is configured to control the circuit switching unit such that the circuit switching unit is in the second state during measurement which is said period of measurement, wherein the physical amount transformer comprises a connection portion (21, 22) used to take out an alternating current, the measuring circuit comprises a detector configured to detect the current, a current input terminal of the detector is connected to one end of the connection portion (21, 22), the rectifier circuit is connected onto a current output terminal side of the detector, and the circuit switching unit is connected between the current output terminal side of the detector and the rectifier circuit.