A command for incrementing a numeric value is inputted by pressing a "2" key. A command for decrementing the numeric value is inputted by depressing an "8" key. At least one of a process for incrementing the numeric value by the amount of increment according to the length of time during which the "2" key is held down and a process for decrementing the numeric value by the amount of decrement according to the length of time during which the "8" key is held down is performed to determine a value to be input.