An oxygen supply device supplying a user with an oxygen gas for inhalation acquires information of the user on percutaneous arterial oxygen saturation (SpO2) using a sensor unit, and calculates, from SpO2, the first moving average value and the second moving average value calculated over a time span longer than the first moving average in a control unit. The control unit calculates a Dip frequency during predetermined time from the SpO2 information, and when the calculated frequency is equal to or larger than the first threshold, the control unit switches the control from the control based on the first moving average value of SpO2 to the control based on the second moving average value.