PURPOSE: To measure the flow rate of slag accurately at all times without any relation to the sticking of slag, by finding the flow rate from non-contact measurement of the width and thickness of the current of slag flowing down from a first channel to a second channel, and of the speed of the current thereof.

CONSTITUTION: A difference in level is provided between a first spout 4A and a second spout 4B, and a non-contact type thickness detector 5 is provided on the lateral side to the part So of a cascade-shaped downfall current of melted slag S flowing down from the first spout 4A to the second spout 4B, so that it watches a prescribed position. In addition, a width detector 6 is provided in front of the part So so as to watch the same position. For the detectors 5 and 6 line sensor cameras are employed, for instance. They perform image processing of signals and detect the thickness d and width W of the downfall current So of the slag at the prescribed position, wherefrom a sectional area A is known. In order to know the speed of the current of slag, a marker Ma having different brightness from that of the melted slag S is dropped in the upper stream of the part of the first gutter 4A. A marker detector 7 is arranged above a position located downstream by a distance (l) from the position whereat the marker is dropped, and thereby the arrival of the marker Ma is monitored. The speed is found by dividing the distance (l) by the difference in time.