A silicon nitride-melilite composite sintered body comprising: silicon nitride crystalline phases; melilite crystalline phases of Me 2 Si 3 O 3 N 4, where "Me" denotes a metal element combining with silicon nitride to generate a melilite; and grain boundary phases; and an additive selected among:: group 2 elements, or: La, Ce, and Pr wherein a ratio of the melilite crystalline phases is not less than 20 area% on a cross section of the silicon nitride-melilite composite sintered body; wherein the silicon nitride-mellilite composite sintered body contains Al in a range of 2 to 10 mol% in oxide equivalent; and wherein: in the case of using at least one element selected among group 2 elements as the additive, the resulting silicon nitride-melilite composite sintered body contains Si in a range of 41 to 79 mol% in Si 3 N 4 equivalent, Me in a range of 13 to 46 mol% in oxide equivalent, and the group 2 element in a range of 5 to 20 mol% in oxide equivalent; or in the case of using at least one element selected among La, Ce, and Pr as the additive, the resulting silicon nitride-melilite composite sintered body contains Si in a range of 45 to 83 mol% in Si 3 N 4 equivalent, Me in a range of 15 to 49 mol% in oxide equivalent, and the at least one element selected among La, Ce, and Pr in a range of 0.3 to 12 mol% in oxide equivalent. silicon nitride crystalline phases; melilite crystalline phases of Me 2 Si 3 O 3 N 4, where "Me" denotes a metal element combining with silicon nitride to generate a melilite; and grain boundary phases; and an additive selected among:: group 2 elements, or: La, Ce, and Pr: group 2 elements, or: La, Ce, and Pr wherein a ratio of the melilite crystalline phases is not less than 20 area% on a cross section of the silicon nitride-melilite composite sintered body; wherein the silicon nitride-mellilite composite sintered body contains Al in a range of 2 to 10 mol% in oxide equivalent; and wherein: in the case of using at least one element selected among group 2 elements as the additive, the resulting silicon nitride-melilite composite sintered body contains Si in a range of 41 to 79 mol% in Si 3 N 4 equivalent, Me in a range of 13 to 46 mol% in oxide equivalent, and the group 2 element in a range of 5 to 20 mol% in oxide equivalent; or in the case of using at least one element selected among La, Ce, and Pr as the additive, the resulting silicon nitride-melilite composite sintered body contains Si in a range of 45 to 83 mol% in Si 3 N 4 equivalent, Me in a range of 15 to 49 mol% in oxide equivalent, and the at least one element selected among La, Ce, and Pr in a range of 0.3 to 12 mol% in oxide equivalent. in the case of using at least one element selected among group 2 elements as the additive, the resulting silicon nitride-melilite composite sintered body contains Si in a range of 41 to 79 mol% in Si 3 N 4 equivalent, Me in a range of 13 to 46 mol% in oxide equivalent, and the group 2 element in a range of 5 to 20 mol% in oxide equivalent; or in the case of using at least one element selected among La, Ce, and Pr as the additive, the resulting silicon nitride-melilite composite sintered body contains Si in a range of 45 to 83 mol% in Si 3 N 4 equivalent, Me in a range of 15 to 49 mol% in oxide equivalent, and the at least one element selected among La, Ce, and Pr in a range of 0.3 to 12 mol% in oxide equivalent.