A film forming method includes supplying a first source gas containing a first metal element onto a substrate, supplying a second source gas containing a second metal element onto the substrate, supplying a reaction gas converted into plasma and containing a nonmetal element reacting with the first metal element and the second metal element to generate a first reaction product and a second reaction product, respectively, to the substrate, to generate a third reaction product containing the first metal element, the second metal element and the nonmetal element. A mixing ratio of the first metal element contained in the third reaction product is higher than that of the second metal element, and a crystallization temperature of the second reaction product is higher than that of the first reaction product.