In order to extract a part of a three-phase flow from a pipe line, a pair of communication pipes, which are connected to upstream and downstream sides of an orifice provided in the pipe line, and a gas-liquid extraction tank are used. Flowing of a slag flow or the like into the pipe line causes a periodical change in a pressure difference between input and output sides of the orifice. This causes extraction of gas-liquid and discharge mainly of gas to be simultaneously effected in the pair of communication pipes and the gas-liquid extraction tank. Within the gas-liquid extraction tank, the gas-liquid is forcibly shaken horizontally, vertically, etc. for agitating, whereby gas-liquid of 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. The mixture liquid flows from the liquid storage tank in an amount required at a Coriolis meter. As a result, high precision density measurement is performed at the Coriolis meter.