A quasi-stagnant coke bed formed in the lower part of blast furnace, called 'deadman', is replaced by new coke over a long renewal interval. The repetition motion, floating of the hearth coke bed due to buoyancy during storage of the molten material and refilling the bottom space due to descending motion of the coke bed during the discharge, is considered to be one of the driving forces for the deadman renewal motion. The purpose of the present study is to clarify the critical condition for the hearth bed to be forced to float up due to buoyancy. Assuming deadman to be a conical body, the stresses acting at the deadman surface are obtained at first extending Walters' theory (Walters, Chem. Eng. Sci. 28 (1973a) 13; 28 (1973b) 779) to the diverging shaft and the converging annular flow-channel between deadman and furnace wall under gas flow. In the next place, a stress field in an active state of stress is proposed to predict the distribution of solid load developed in the deadman and lower hearth part. The critical storage level of molten liquid for hearth bed floating is then derived from a force balance between the solids load, buoyancy and wall shear stress. It is found that the floating mode depends on the horizontal profile of vertical load at the critical liquid level. In the case of the load profile is uniform across the cross section, the hearth bed would start to float up leaving a solid-free liquid space with horizontally uniform height. In the case of the vertical load is high in most of the cross section, but becomes lower near the wall, two types are possible for bed motion. Thus, a diagram classifying the floating mode into three categories according to the way the hearth bed behaves at the beginning of floating is given.