PURPOSE: To enable the accurate measurement of a flow rate by using a vertical orifice, and thereby making the pressure on the vertical section thereof constant.

CONSTITUTION: An orifice 2 of a straight pipe 1 forming a flowmeter is formed vertically, and thereby the pressure on each part of each section of the orifice is turned to be constant differently from the case of a horizontal orifice. Accordingly, the Bernoulli's theorem on a fluid in a steady state is applied excellently, and thus a flow rate can be measured accurately through a liquid head difference measuring tube 3.