An operation controlling apparatus for a passenger operated machine such as a tractor, snowmobile or the like. The apparatus includes an engine cut-off mechanism which terminates engine operation if an operator is expelled from the machine seat for a pre-determined time. Relatively short, intermittent periods of displacement of an operator from the seat, such as when travelling over rough or bumpy terrain, will not terminate engine operation. After emergency engine stoppage, the apparatus requires the machine be placed in a start waiting state, for instance by disengaging the clutch or when the transmission is placed in neutral, before the engine can be re-started.