In a system for preventing an equipment such as lawnmower from theft, there are equipped with an internal combustion engine that is mounted on the equipment and is equipped with a generator that generates electric operating power when the engine is operated, a recoil starter to be manipulatable by an operator to operate the engine, an electronic key that is adapted to be carried by the operator and stores authenticating data for identifying the operator who carries the electronic key; and an authenticator that is installed at the equipment and performs authentication check as to whether the electronic key is an authorized key using stored key-checking data, when the authenticating data is outputted from the electronic key. In the system, the authenticator performs the authentication checkwhen the electric operating power is supplied from the generator, thereby enabling to be installed in battery-less equipment and ensure to prevent the equipment from theft.