A constituent concentration measuring apparatus characterized by comprising: light generating means adapted to generate light; light outgoing means adapted to output light toward a test subject; light modulation means adapted to electrically intensity modulate the light generated by the light generating means at a constant frequency, the light outgoing means being adapted to output the intensity modulated light toward the test subject; an acoustic wave generator adapted to output an acoustic wave toward the test subject, said light outgoing means being fixed to said acoustic wave generator so as to keep the relative position to said acoustic wave generator; and acoustic wave detection means adapted to detect a photo-acoustic signal emitted from the test subject, which is irradiated with said intensity modulated light, and said acoustic wave transmitted from said acoustic wave generator through the test subject; characterised in that drive means are provided that are adapted to vary a position of at least one of said acoustic wave generator and said acoustic wave detection means, and control means are provided that adapted to control said drive means by outputting a control signal to the drive means, so that at least one of said acoustic wave generator and said acoustic wave detection means is moved, said control means being further adapted to determine a position of said acoustic wave generator or said acoustic wave detection means, where the intensity of the acoustic wave output from said acoustic wave generator and detected by said acoustic wave detection means becomes a particular value, such that the photo-acoustic signal propagates through the propagation path in which the acoustic wave intensity detected by the acoustic wave detection means becomes the particular value. light generating means adapted to generate light; light outgoing means adapted to output light toward a test subject; light modulation means adapted to electrically intensity modulate the light generated by the light generating means at a constant frequency, the light outgoing means being adapted to output the intensity modulated light toward the test subject; an acoustic wave generator adapted to output an acoustic wave toward the test subject, said light outgoing means being fixed to said acoustic wave generator so as to keep the relative position to said acoustic wave generator; and acoustic wave detection means adapted to detect a photo-acoustic signal emitted from the test subject, which is irradiated with said intensity modulated light, and said acoustic wave transmitted from said acoustic wave generator through the test subject; characterised in that drive means are provided that are adapted to vary a position of at least one of said acoustic wave generator and said acoustic wave detection means, and control means are provided that adapted to control said drive means by outputting a control signal to the drive means, so that at least one of said acoustic wave generator and said acoustic wave detection means is moved, said control means being further adapted to determine a position of said acoustic wave generator or said acoustic wave detection means, where the intensity of the acoustic wave output from said acoustic wave generator and detected by said acoustic wave detection means becomes a particular value, such that the photo-acoustic signal propagates through the propagation path in which the acoustic wave intensity detected by the acoustic wave detection means becomes the particular value.