PURPOSE: To simplify the structure of an engine ignition device and to improve the accuracy thereof by taking advantage of a dividing signal of a dividing circuit that divides a clock signal of a microcomputer, and by switching a switching circuit that is connected to a primary side of a step-up transformer to the secondary side of which an ignition circuit is connected.

CONSTITUTION: When a clock signal stored in a CPU 11 is divided by a dividing circuit 12 and is applied to a base side of a transistor 9, the transistor 9 is turned 'ON' and 'OFF' alternately in a discontinuous way. When the transistor 9 is turned 'ON', electric current runs in a primary side of a step-up transformer 2 from a battery 1, and high voltage is generated on a secondary side thereof. A condenser 5 is also charged through a diode 3. When a control signal is applied from the CPU 11 to the gate of a thyristor 4 at the time of ignition, an electric charge of the condenser 5 is discharged to the ground through the thyristor 4, while primary current runs in the primary side of an ignition coil 6. High voltage is generated on a secondary side of the ignition coil 6, and ignition is thus performed by an ignition plug 7.