The present invention discloses the following procedures: in order to extract a part of a three-phase flow from a pipe line (23), a pair of communication pipes (13), which are connected to upstream and downstream sides of an orifice (12) provided in the pipe line (23), and a gas-liquid extraction tank (14) are used. Flowing of a slag flow or the like into the pipe line (23) causes a periodical change in a pressure difference between input and output sides of the orifice (12). This causes extraction of gas-liquid and discharge mainly of gas to be simultaneously effected in the pair of communication pipes (13) and the gas-liquid extraction tank (14). Within the gas-liquid extraction tank (14), the gas-liquid is forcibly shaken horizontally, vertically, etc. for agitating, whereby gas-liquidof high liquid-phase rate remains. Then, the gas is removed from this gas-liquid of high liquid-phase rate to thereby extract mixture liquid, which is accumulated in a liquid storage tank (17). The mixture liquid flows from the liquid storage tank (17) in an amount required at a Coriolis meter (38). As a result, high precision density measurement is performed at the Coriolis meter (38).