Cast-in insert of gray iron around austenitic stainless steel pipe was made using thermal spraying of a Ni base self-fluxing alloy and pressurized Ar gas. The microstructures and basic properties of the bonded region were investigated and also practicalities and problems with this method were discussed. The experiments showed that the bonding in the cast-in insert was quite sensitive to the volume ratio of the stainless steel pipe and cast iron, pouring temperature, and other casting conditions, still thermal spraying greatly expanded the region where successful bonding was obtained. The microscopic observation showed that the bonding was metallurgically satisfactory and that there were a carburized layer on the steel pipe, a diffusion zone of sprayed alloy and rapidly solidified cast iron structure. Metal-carbon compounds and martensite formation were also observed in the diffusion zone. The strength of the bonded region was proved adequate by punching tests. Thus, thermal spraying was found effective in cast-in insert.