A forming die assembly includes an upper die including a first backup member and a first press member that is attached to the first backup member, has an L shape in a vertical sectional view, and is heat-resistant; and a lower die including a second backup member and a second press member that is attached to the second backup member, has an L shape in the vertical sectional view, and is heat-resistant. The first press member and the second press member press upper and lower surfaces of a workpiece accommodated in a press space that is formed between the first press member and the second press member, so as to produce a forged product.