PURPOSE: To obtain an engine rotation detecting apparatus excellent in noise resistance and high in reliability, by constituting so as to detect the primary side signal of an ignition coil by light.

CONSTITUTION: The primary current of an ignition coil 2, that is, the primary current of current transformer 7 is intermittently flowed every when a point is subjected to opening and closing operation and a current 1/N times of the primary current is flowed through the secondary winding of the current transformer 7. A light emitting element 8 emits light by the aforementioned secondary current and comes to light emitting operation synchronous to the opening and closing operation of the point 3. A resistor 9 is inserted in order to prevent the generation of overvoltage when the secondary side of the current transformer is opened. In the next step, a light receiving element 10 receives the light signal of the light emitting element 8 to convert the same to an electric signal and an electric signal synchronous to the rotation of an engine is obtained at the output terminal of a wave form shaping circuit 11. By the above mentioned constitution, the noise signal of the primary side of the ignition coil 2 does not directly entire the circuit 11 and an engine rotation detecting apparatus high in reliability is obtained.