A nonstop program system includes program sides each including programs accomplishing the required function of the system and a dedicated variable area storing variables used for executing the program. Those programs are independent of each other and have the same contents. When the operational state of one program side is set to an ACT state, while using the one program side as the active side for executing the program in the one program side, the operational state of another program side is set to a HOT_SBY state, and a variable area of the other program side is synchronized with the variable area of the active side, thereby alternately and periodically switching the operational states of the program sides. When switching the active side, the operational state of the other program side is switched to the ACT state to cause the other program side as the active side to continuously execute the program.