In a coupled ring oscillator including q ring oscillators each including p inverter circuits connected together to form a ring shape, and a phase coupling ring including (p×q) phase coupling circuits each of which is configured to couple an output of one of the p inverter circuits of one of the q ring oscillators to an output of one of the p inverter circuits of another one of the q ring oscillators in a predetermined phase relationship, and which are connected together to form a ring shape, for at least one group made up of one of the p inverter circuits in each of the q ring oscillators, outputs of the q inverter circuits belonging to the at least one group are fixed in phase with one another, the q ring oscillators are caused to oscillate in the in-phase fixed state, and then, the outputs of the q inverter circuits are released from the in-phase fixed state.