A divider control circuit outputs a divider control signal that is meant to increase the division ratio from a first division ratio through a second division ratio greater than the first division ratio to a third division ratio greater than the second division ratio and then return the division ratio through a fourth division ratio smaller than the third division ratio to a fifth division ratio smaller than the fourth division ratio and greater than the first division ratio. Moreover, control is exerted so that the absolute value of the time rate of change of the division ratio in the increase from the second division ratio to the third division ratio is smaller than the absolute value of the time rate of change of the division ratio in the increase from the first division ratio to the second division ratio.