a quantitative liquid sampling instrument comprises a main body of a pipet device which includes a pipet tip having an opening at its lower end for sucking and discharging a liquid . the pipet tip being in a shape of a tube shell having a cavity for holding a liquid sucked therein through the opening and being made of a transparent or semi - transparent material . the main body further comprising a device for applying a gas pressure for sucking or discharging the liquid to or from the inside of the cavity of the pipet tip . the instrument further comprising a liquid level detecting device installed under the main body of the pipet device ; a vertical movement control device for moving the main body of the pipet device or a container of the liquid to make them relatively approach one another until a predetermined length of the lower end of the pipet tip is dipped into the liquid in the container ; and a computing device for computing the end point of relative approaching movement of the main body of the pipet device and the container and for outputting a signal regarding the end point to the vertical movement control device .