A method of controlling a semiconductor device using a microcomputer having a timer is provided. An interrupting step interrupts the semiconductor device using the timer, by generating a control signal for controlling a timing of turning on/off the semiconductor device. The interrupting step is performed in accordance with an interrupting signal which is input to the microcomputer at every occurrence of a predetermined period. The interrupting step comprises a setting step of giving a set value to the timer and a calculating step of determining a set value for a subsequent interrupting step, such that the set value determined in the calculating step is given to the timer in the setting step of the subsequent interrupting step. Therefore, a longer time can be allowed for determining a set value for the timer, and the processing speed required for the microcomputer can be reduced.