In a combined scanning near field optical microscope (Nsom) and atomic force microscope (AFM), an optical fibre probe (1) which has a minute opening on the top of a sharpened tip is brought close to a sample (2), and the probe is moved by a piezo actuator (15) along x- and y-axis directions so that a minute spot beam emanating from the minute opening can scan over the sample. For circular polarisation modulation to be incorporated in the process, a beam is given an optical delay, before it is incident on the optical fibre probe (1), changing at a frequency of p (Hz) by means of a piezo-optical modulator (10). A minute spot beam emanating from the minute opening passes through the sample (2) to be received after passage through the sample (2) to be received after passage through an analyser (5) by a light receiving element (7). The output from the light receiving element (7) is fed to a lock-in amplifier, p- and 2p-components are separated through lock-in rectification, and they are rendered into images by a controller (16). It is used for measuring the distribution of magneto-optical effects.