Abstract: A fixture is provided on a set probe and a hole of the set probe communicates with that of the fixture so that a measuring probe consisting of an optical fiber bundle is inserted in the holes. A measured object is irradiated with light from a forward end of the measuring probe, so that outgoing light from the measured object enters the measuring probe. An air pack for coming into contact with the measured object is provided on the forward end of the measuring probe in a position not hindering emission and incidence of light. The air pack is connected with a pressure sensor, so that an indicator reads the pressure.