An apparatus which can detect leak of a liquid injected through an injection needle into a blood vessel of a human body near its surface with high accuracy. The apparatus emits a light ray in a predetermined wavelength band containing a particular wavelength at which the reflectivity for the liquid is lower than the reflectivity for internal tissues of the human body, and detects the light ray in the wavelength band reflected inside the human body. The leak of the liquid can be detected without being affected by variations in the human body or physical condition.
