A head restraint position adjusting device, comprising: at least three sensing electrodes (11 - 15) provided side by side in a height direction in a head restraint provided on a seat of a vehicle and configured to sense capacitances between a human body sitting on the seat and the head restraint; a detection circuit configured to detect an estimated center position of a head of the human body in the height direction based on sensing signals from the at least three sensing electrodes; and position adjusting means configured to, in accordance with a detection result from the detection circuit, adjust a position of the head restraint relative to the seat based on the estimated center position; characterized in that in a case (a) where there is any sensing electrode that indicates a larger capacitance value than that of its adjoining sensing electrode located under it among sensing electrodes located above a largest value sensing electrode, which has sensed a capacitance indicating a largest capacitance value among capacitance values of capacitances sensed by the respective sensing electrodes, the detection circuit being configured to detect as the estimated center position, a position that horizontally corresponds to a sensing electrode that has sensed a capacitance indicating a largest capacitance value among capacitance values of capacitances sensed by any such sensing electrodes that indicate a larger capacitance value, and in any other case than the case (a), the detection circuit being configured to detect a position that horizontally corresponds to the largest value sensing electrode as the estimated center position. at least three sensing electrodes (11 - 15) provided side by side in a height direction in a head restraint provided on a seat of a vehicle and configured to sense capacitances between a human body sitting on the seat and the head restraint; a detection circuit configured to detect an estimated center position of a head of the human body in the height direction based on sensing signals from the at least three sensing electrodes; and position adjusting means configured to, in accordance with a detection result from the detection circuit, adjust a position of the head restraint relative to the seat based on the estimated center position; characterized in that in a case (a) where there is any sensing electrode that indicates a larger capacitance value than that of its adjoining sensing electrode located under it among sensing electrodes located above a largest value sensing electrode, which has sensed a capacitance indicating a largest capacitance value among capacitance values of capacitances sensed by the respective sensing electrodes, the detection circuit being configured to detect as the estimated center position, a position that horizontally corresponds to a sensing electrode that has sensed a capacitance indicating a largest capacitance value among capacitance values of capacitances sensed by any such sensing electrodes that indicate a larger capacitance value, and in any other case than the case (a), the detection circuit being configured to detect a position that horizontally corresponds to the largest value sensing electrode as the estimated center position.