An inspection method using a portable optical inspection apparatus adapted to be driven by a battery and capable of producing an accurate sensing result without being adversely affected by the environmental temperature conditions is disclosed. The light is radiated on a sensor chip for a predetermined length of time and the amount of a substance contained in a specimen placed on a sensing thin film is measured by an optical signal obtained from the sensing chip, using an optical inspection apparatus comprising the sensor chip including a substrate 5, an optical waveguide layer 6 arranged on the substrate and the sensing thin film 7 attached on the surface of the optical waveguide layer, a light source 8 for radiating the light on the sensor chip and a photodetector 9 for receiving and converting the light output from the sensor chip into an electrical signal The light source is turned on and off a plurality of times for a predetermined length of time during which the inspection light is radiated. The amount of light with the light source on is measured by the photodetector thereby to determine the amount of the substance. In the process, the light amount with the light source off can also be measured so that the amount of the substance is determined from the difference between the light amount with the light source on and off.