An endoscope apparatus and method are disclosed wherein first and second light beams, each having different center wavelengths, illuminate an object. A detector and a light receiver optical system are provided that receive back-scattered light from first and second illumination devices. A processor is provided that calculates a value corresponding to the size of particles that back-scatter light of the first and second light beams that are incident onto the object, with the calculated value being independent of the concentration of the particles. The first illumination device and the second illumination device are arranged in a specified manner so that a specified condition is satisfied.