An image forming apparatus has a clock supply circuit supplying a signal at a first clock frequency or a second clock frequency lower than the first frequency, a first CPU performing image data processing based on the signal, and a second CPU measuring time based on the signal to determine execution timing of each process performed in control processing. The image forming apparatus includes a load determination unit determining whether a load of the image data processing is higher than a reference level, and a control unit that switches the frequency to the first frequency when the load is higher than the reference level, and to the second frequency when the load is not higher. When the frequency switching occurs, the second CPU changes a method for measuring time. The second CPU completes one process before the switching and starts the next one after the switching.