To quantitatively measure the water loss of wall surface in a blast furnace in a real time after lowering a stock level of charged material in the blast furnace.

At the time of measuring the wear loss of the wall surface 2 in the blast furnace 1, when the stock level of the charged material 6 in the furnace is lowered, a pipy lance 7 is vertically lowered in the furnace so as to be freely swung and elevated/lowered. Under condition of facing the tip end part of this pipy lance 7 to the measuring position of the wall surface 2 in the furnace, laser beam is projected downward from a laser beam range finder 9 arranged at the upper end part of the pipy lance 7 and an optical path is bent to the right angle with a reflector 8 arranged at the lower end part and irradiates the wall surface 2 in the furnace. The laser beam reflected on the wall surface 2 in the furnace is received to the laser beam range finder 9 through the reflector f8 and the distance format he projecting position in the lance to the wall surface 2 in the furnace is measured. The wear loss of the refractory of the wall surface 2 in the furnace is quantitatively measured from this measured distance and the geometric positional relation between the pipy lance 7 and the blast furnace body.