To provide an inexpensive ignition system with small number of items and processing steps which enables normal and reverse rotation of an engine.

A rotational body 10 is attached to a crankshaft. The rotational body 10 is provided with teeth 20, 21, 22, 23, 24, 25 projected outward in a diameter direction. The tooth 20 is longer than the other in a rotational direction. The teeth 20 and 21 are arranged across a top dead center between a compression process and an explosion process of a first cylinder, the teeth 22 and 23 across a top dead center between the compression process and the explosion process of the second cylinder, the teeth 24 and 25 across a top dead center between the compression process and the explosion process of the third cylinder, respectively separated from the top dead centers by the same angles in a rotational direction. A timing sensor 30 outputs positive and negative signals in rear and front sides of a rotational direction of each tooth. In a control circuit of a control unit 31, a rotational front signal G and a rotational rear signal G2 are generated as pulse signals from the signals output from the timing sensor 30. A rotational direction of an engine can be determined from the generation intervals of the signals G1 and G2.