A power supply device 3 including: a current source 31 for supplying current to a load circuit 4 which operates on the supplied current, and a power switch unit 32 between the current source 31 and the load circuit 4 to control passage of current, wherein: the power switch unit 32 includes a switching element 323 connected in parallel with the open-close element 321, a power supply controller 33 including an open-close element controller 31 which controls operation of the open-close element 321 and a switching element controller 332 which controls operation of the switching element 323, and a voltage detector 34 which detects terminal voltage of the load circuit 4; and the power switch unit 32 turns on the switching element before supplying current to the load circuit and closes the open-close element when the terminal voltage of the load circuit becomes almost equal to supply voltage.