In an outboard motor having a prime mover, a gear mechanism transmitting a driving force of the prime mover to a propeller, and a gear case housing the gear mechanism, it is configured to have a drain bolt fastened to a drain hole bored at the gear case and constituted as a magnetic member; a stacked portion provided at an exposed part of the drain bolt in an inside of the gear case and having a conductor, an insulator and a resistive element partially making contact with the conductor that are stacked in a gravitational direction; a current detector detecting a current value conducted from the conductor to the drain bolt under a condition where voltage is applied to the conductor; and a wear debris amount detector adapted to detect an amount of wear debris of the gear mechanism deposited at the drain bolt based on the conducted current value.