To provide an inexpensive power supply device and a power supplying method for preventing an unfair use.

Current from a power source 30 becomes excess a current beyond a prescribed current value because resistance 14 within an IG key cylinder switch 12 is not interposed, even if an engine is started by directly connecting a power source line from a YASFET 40 being a switching device and a relay ST26 without interposing the resistance 14. The YASFET 40 detects that current from the power source 30 becomes excess current beyond a prescribed current value and current to be supplied to the relay ST26 is interrupted by the YASFET 40. Therefore, an unfair use can be prevented because the engine is not started by the direct connection. The power supply device and power supplying method which are not complicated in constitution and inexpensive as compared with a case that a security system using a microcomputer is jointly used.