a crank angle sensor comprises a rotor plate formed with first slits for detecting a first cylinder angular position , second slits for a second cylinder angular position , and a third slit for detecting an angular position of a specified cylinder . the first slits are equi - angularly distant one after another and arranged within a predetermined annular band within the rotor plate , and the second slits are equi - angularly distant one after another and arranged within the predetermined annular band . each of the first slits is located adjacent to the corresponding one of the second slits . in an ignition timing control system using the crank angle sensor , the control is performed by generating a train of pulses including a train of leading pulses and a train of trailing pulses superimposed to each other , determining whether a start switch is turned on or not , determining whether an engine temperature is greater than a predetermined value or not , and carrying out an ignition at a falling edge of each of the trailing pulses when the engine temperature is greater than the predetermined value , and at a falling edge of each of the leading pulses when the engine temperature is not greater than the predetermined value .