Antitheft apparatus for equipment with prime mover

In an apparatus for preventing theft of equipment having a prime mover, an prime mover controller, and an authenticator that acquires ID information from an electronic key when the key is brought close thereto by an operator (S10 S12), and permits the prime mover controller to start the prime mover when the acquired ID information is determined to correspond with authentication ID information (S14 S18), data indicating a number of times the key is updated is included in the ID information, and the authenticator determines whether the number of times indicated in the data is greater than that in the authentication ID information (S20), and when the number of times indicated in the data is greater than that in the authentication ID information, updates the authentication ID information such that it is equal to the number of times indicated in the data (S22), thereby enabling to easily update authentication ID information of the equipment.