An observation apparatus, comprising: a confocal imaging system having a laser light source that is a coherent light source configured to irradiate an observation target with excitation laser light, a scanning mirror configured to scan the observation target with the excitation light, and a bandpass filter configured to receive back-reflected light from the observation target, to transmit Rayleigh-scattered light from the observation target and to filter out light other than Rayleigh-scattered light; a detecting unit configured to receive Rayleigh-scattered light from the observation target via the confocal optical system, the light including light derived from χ and light derived from χ, and to detect light derived from χ while at least partially blocking the light derived from χ by causing the waves of the light derived from χ, being coherent light and having the same wavelength as the light of the coherent light source, to weaken each other through interference; and a processing unit configured to generate an image based on the light derived from χ detected by the detecting unit, wherein χ is linear susceptibility and wherein χ is third-order non-linear susceptibility of the observation target, and wherein the detecting unit has: at least one light splitting unit arranged on an optical path of the light obtained from the observation target and configured to split the light into beams of light; and at least two mirrors, and wherein the detecting unit at least partially blocks the light derived from χ by utilizing interference occurring when the beams of light reflected by the at least two mirrors are combined. a confocal imaging system having a laser light source that is a coherent light source configured to irradiate an observation target with excitation laser light, a scanning mirror configured to scan the observation target with the excitation light, and a bandpass filter configured to receive back-reflected light from the observation target, to transmit Rayleigh-scattered light from the observation target and to filter out light other than Rayleigh-scattered light; a detecting unit configured to receive Rayleigh-scattered light from the observation target via the confocal optical system, the light including light derived from χ and light derived from χ, and to detect light derived from χ while at least partially blocking the light derived from χ by causing the waves of the light derived from χ, being coherent light and having the same wavelength as the light of the coherent light source, to weaken each other through interference; and a processing unit configured to generate an image based on the light derived from χ detected by the detecting unit, wherein χ is linear susceptibility and wherein χ is third-order non-linear susceptibility of the observation target, and wherein the detecting unit has: at least one light splitting unit arranged on an optical path of the light obtained from the observation target and configured to split the light into beams of light; and at least two mirrors, and wherein the detecting unit at least partially blocks the light derived from χ by utilizing interference occurring when the beams of light reflected by the at least two mirrors are combined. at least one light splitting unit arranged on an optical path of the light obtained from the observation target and configured to split the light into beams of light; and at least two mirrors, and wherein the detecting unit at least partially blocks the light derived from χ by utilizing interference occurring when the beams of light reflected by the at least two mirrors are combined.