This breathing function examining instrument (1, 2, 3, 50, 70) measures the internal pressure of the oral cavity of a subject which varies with breathing and is provided with a main body (5, 6), a resistance setting unit (40, 60, 80) and a pressure measuring unit (32). The main body (5, 6) is formed in a tubular shape and has an air inlet which is a hole through which air flows into the tube from the outside. The main body (5, 6) is further provided at one end with a one-way valve (11) for preventing air from flowing thereinto from the outside and allowing the subject to intake air through the other end without a one-way valve. The resistance setting unit (40, 60, 80) variably sets the degree of inflow resistance to which air flowing into the main body is subjected. The pressure measuring unit (32) measures the pressure of air flowing in the tube of the main body as an oral cavity internal pressure.
