A measuring unit is placed at the surface of a living body. The pressure inside of a glass container is reduced by a pressure reducing tank, so that a subcutaneous exuding fluid is taken out at the surface of the living body from under a skin. The subcutaneous exuding fluid is allowed as a sample to be measured to reach a metallic film through porous molecular weight selective films. A light beam emitted from a light source passes through a prism, and then, is reflected on the metallic film, and finally, is received on a linear array sensor. A minimum intensity position on the linear array sensor, generated by surface plasmon resonance is obtained, and then, a resonant angle is detected. The concentration of substance to be measured contained in the sample to be measured is calculated based on the resonant angle and position. Thus, the concentration of a component contained in a bodily fluid can be readily measured for a long time of period in a continuous manner without any special treatment by taking a very small quantity of exuding fluid without any pain.