Abstract: An apparatus includes a stimulation part that stimulates a brain of an object; a sensing part that measures a response of the brain to a stimulation; and a control part that determines whether a first response entraining oscillation synchronized in a plurality of brain areas of the object is derived or not, based on a measured brain response. The stimulation part is an electrical stimulation part that transfers a transcranial alternating current stimulation (tACS) and the transcranial alternating current stimulation is a first combined stimulation in which a signal is repeatedly ON/ OFF according to a predetermined first frequency and an ON signal according to the first frequency is applied as a stimulation according to a predetermined second frequency.