PURPOSE: To quickly and accurately measure the whole profile of a charge in a furnace by using a measuring instrument having a simple structure without requiring any large-scale machine device and without receiving any influence from the high-temperature dusty atmosphere in the furnace.

CONSTITUTION: A microwave probe 9 is pivotally set in a rotatable state in a case 8 provided at an outside-furnace position apart from the furnace center axis of the bell-less furnace top section of a blast furnace. By continuously radiating frequency-modulated microwaves 12 upon the surface of a charge 5 in the furnace within a diametral extent passing through the furnace core or along the circumferential direction around the furnace core, the distance to the surface of the charge is measured from the time lag between the transmission and reception of the microwaves 12 by receiving reflected waves from the surface of the charge 5.