To provide a process for producing a gradient casting having a transition structure which is simple in production stages and is good in casting plan and yield and its casting mold.

A reaction chamber 5 is formed via a shutter member 7 in a runner 3 for introducing molten metal 8 into a casting section 4 and the molten metal 8 is poured into the casting section 4 through this runner 3, by which a first casting 10 is formed. The shutter member 7 is thereafter opened to allow the molten metal 8 to flow into the reaction chamber 5 where the molten metal 8 and a molten metal reacting agent 6 packed into the reaction chamber 5 are reacted to form the reacted molten metal 9. This reacted molten metal 9 is poured into the remaining gap in the casting section 4, by which a second casting 11 is formed.