In an apparatus for preventing theft of plural, such as two, outboard motors (12F, 12S) adapted to be mounted on a boat, each of the outboard motors having an engine (16F, 16S), an engine controller (24F, 24S), and an authenticator (26F, 26S) that acquires ID information (A, B) from an electronic key (30F, 30S) when the key is brought close thereto by an operator, and gives permission to the engine controller to start the engine when the acquired ID information corresponds with the authentication ID information, and communicates with the engine controller of other outboard motor to notify that the permission to start the engine was given, thereby enabling the operator to easily start the outboard motors installed on one boat and having their respective authenticators.