The device of the invention is equipped with a pressure sensor and a control unit, and the control unit judges a point at which a pressure gradient calculated from a signal of the pressure sensor becomes larger in the absolute value than a pressure gradient threshold as an inspiration sensing point and starts respiratory gas supply. In addition, the device judges from a frequency of the inspiration sensing point during predetermined time whether a pressure gradient threshold used as a criterion for the inspiration sensing point corresponds to an activity state of the user, and when the pressure gradient threshold does not correspond to the activity state, switches the pressure gradient threshold to another threshold and thus fits the pressure gradient threshold to the activity state.