Why does the engine won't shut off?

I have a 1997 toyota 4runner V6 3.4lts. The car starts fine, but when I try to shut the engine off, starter motor kicks on, and tries to start the engine again ; even, without the key on the ingnitions switch.I am able to hear the engine cranking as many times until I put back the key into the ignition switch and I flip the switch. I have to disconnect both battery terminal in order to kill the engine.