To grasp the worn state of refractory of a furnace wall in a real time after lowering the stock level of charged material in a blast furnace and to quantify the worn quantity.

At the time of grasping the refractory worn state of the furnace wall 23 constructed in the blast furnace 1, several kinds of refractory layers having different colors are constructed distributedly in the thickness direction o the furnace wall 23 as the refractory of the furnace wall 23 so that the furnace wall is beforehand constructed by using the laminated colored refractories 13. After lowering the stock level of the charged raw material 4 in the furnace, the inner surface of the refractory of the furnace wall 23 is observed with an observing camera 10 and the worn state of the furnace wall refractory is grasped based on the changing state of the color caused by wearing of the colored refractory 13 and also, this worn quantity is quantified. Since not only the position where the furnace wall refractory in the blast furnace is worn can surely be specified, but also, this worn quantity can be quantified, the repair or the furnace wall 23 can be executed in good timing and the extension of service life of the furnace wall refractory in the blast furnace can be achieved.