tion means is z0, the wavelength of the illuminating light is xcex, a maximum value of a diameter of the probe is dmax within a range from a top end of the probe to a distance z0, and the top end diameter of the probe is d0, the diameter of the probe monotonously increases from the top end to the distance z0, and dmaxxe0x00xa0d0(z0+xcex/0)/(xcex/0) is given. Also, to achieve the above object, according to the present invention, there is provided a near-field optical microscope comprising: light illumination means for illuminating a sample surface of a sample with light; a probe having a top end provided at a position near the sample surface which is illuminated with the light; light detection means for detecting scattered light scattered by the probe; and scanning means for scanning the sample and the probe relatively to each other, wherein a top end diameter of the prove is equal to or smaller than xc0xc of a wavelength of the light illuminating the probe, and where a length of a 