A signal processing apparatus for processing a periodic signal outputted from a signal source has a central processing unit and a task switch timer. The central processing unit performs multiple tasks including a signal processing task in parallel. In the signal processing task, the central processing unit starts to process the periodic signal after performing a synchronization processing to synchronize with the periodic signal, setting the task switch timer to a predetermined time upon completion of the synchronization processing, and enabling an interrupt to the central processing unit upon completion of the synchronization processing. The task switch timer disables the interrupt to the central processing unit immediately before expiring. The task switch timer outputs a task switch signal to the central processing unit when expiring, so that the central processing unit switches to the signal processing task.