An authentication method for a controller, a first device, and a second device performed in an authentication system in which the controller, the first device, and the second device share a group key (3040, 3050, 3140, 3150), the authentication method comprising: a first mutual authentication step (S102, S1020, S1120) of performing mutual authentication, creating a group key (3040, 3050), and sharing the group key (3040, 3140) by the controller and the first device; a second mutual authentication step (S143, S148, S1040, S1140) of performing mutual authentication by the controller and the second device, and making the second device also share the group key (3040, 3140) created in the first mutual authentication step (S102, S1020, S1120); a third mutual authentication step (S303, S4030, S4120) of, after the second mutual authentication step (S148, S1040, S1140), performing mutual authentication again, updating the group key, and sharing the updated group key (3050, 3150) by the controller and the first device; and a fourth mutual authentication step (S308, S4030, S4120) of performing mutual authentication by the controller and the second device, and making the second device also share the updated group key (3050, 3150); characterized in that the controller simultaneously transmits information encrypted by using the group key (3040, 3050, 3140, 3150) to the first device and the second device; the first mutual authentication step (S102, S1020, S1120) further sets the first device as a reference device, such that the first device is the reference device and the second device is not the reference device; the fourth mutual authentication step (S308, S4030, S4120) is performed at a group key update timing when the controller and the first device, which is the reference device, update the group key, and by a transmitting step (S311, S4160) of simultaneously transmitting a group key update notification from the controller to the first device and the second device if the third and the fourth authentication steps (S303, S308) are both completed, so as to perform group key switching (S312, S313, S314) from the group key (3040, 3140) to the updated group key (3050, 3150) at the controller and the first and the second device (200a, 200b) at the same timing. a first mutual authentication step (S102, S1020, S1120) of performing mutual authentication, creating a group key (3040, 3050), and sharing the group key (3040, 3140) by the controller and the first device; a second mutual authentication step (S143, S148, S1040, S1140) of performing mutual authentication by the controller and the second device, and making the second device also share the group key (3040, 3140) created in the first mutual authentication step (S102, S1020, S1120); a third mutual authentication step (S303, S4030, S4120) of, after the second mutual authentication step (S148, S1040, S1140), performing mutual authentication again, updating the group key, and sharing the updated group key (3050, 3150) by the controller and the first device; and a fourth mutual authentication step (S308, S4030, S4120) of performing mutual authentication by the controller and the second device, and making the second device also share the updated group key (3050, 3150); characterized in that the controller simultaneously transmits information encrypted by using the group key (3040, 3050, 3140, 3150) to the first device and the second device; the first mutual authentication step (S102, S1020, S1120) further sets the first device as a reference device, such that the first device is the reference device and the second device is not the reference device; the fourth mutual authentication step (S308, S4030, S4120) is performed at a group key update timing when the controller and the first device, which is the reference device, update the group key, and by a transmitting step (S311, S4160) of simultaneously transmitting a group key update notification from the controller to the first device and the second device if the third and the fourth authentication steps (S303, S308) are both completed, so as to perform group key switching (S312, S313, S314) from the group key (3040, 3140) to the updated group key (3050, 3150) at the controller and the first and the second device (200a, 200b) at the same timing.