A method for producing a MgF 2 sintered body for a radiation moderator comprising MgF 2 having a compact polycrystalline structure with a bulk density of 3.07g/cm 3 or more and a relative density of 97.5% or more, which has a bending strength of 12 MPa or more and a Vickers hardness of 100 or more as regards mechanical strength, comprising the steps of: pulverizing a MgF 2 raw material with a purity of 99.9 % by weight or more to control the particle size, so as to allow the maximum particle diameter in a particle size distribution to be 50 μ m or less, the shape of the particle size distribution curve to be of sub-1-peak type or 1-peak type as defined in the description, and the median diameter to be 6 μ m or less; adding 0.02-1% by weight of a sintering aid to the particle-size-controlled raw material to mix; molding the compound at a molding pressure of 5MPa or more using a press molding device; molding the press molded article at a molding pressure of 5MPa or more using a cold isostatic pressing device; conducting preliminary sintering by heating the CIP molded article in a temperature range of 600°C-700°C for 4 - 10 hours in an air atmosphere (preliminary sintering step); conducting atmospheric sintering or pressure sintering by heating in a temperature range from (Tn-100)°C to (Tn)°C, that is 750°C - 850°C when the starting temperature of foaming of the preliminary sintered body is 850°C, for 3-12 hours in an air atmosphere or in an inert gas atmosphere or in a vacuum atmosphere (primary sintering step); forming a sintered body having a compact structure by heating subsequently to the heating in the preceding step in a temperature range of 900°C-1150°C for 0.5 - 8 hours under atmospheric pressure or under pressure in the same atmosphere as the preceding step (secondary sintering step); cooling the sintered body after the secondary sintering step; and conducting tertiary sintering by reheating in a temperature range of 900°C -1150°C under pressure in an inert gas atmosphere or in a vacuum atmosphere. pulverizing a MgF 2 raw material with a purity of 99.9 % by weight or more to control the particle size, so as to allow the maximum particle diameter in a particle size distribution to be 50 μ m or less, the shape of the particle size distribution curve to be of sub-1-peak type or 1-peak type as defined in the description, and the median diameter to be 6 μ m or less; adding 0.02-1% by weight of a sintering aid to the particle-size-controlled raw material to mix; molding the compound at a molding pressure of 5MPa or more using a press molding device; molding the press molded article at a molding pressure of 5MPa or more using a cold isostatic pressing device; conducting preliminary sintering by heating the CIP molded article in a temperature range of 600°C-700°C for 4 - 10 hours in an air atmosphere (preliminary sintering step); conducting atmospheric sintering or pressure sintering by heating in a temperature range from (Tn-100)°C to (Tn)°C, that is 750°C - 850°C when the starting temperature of foaming of the preliminary sintered body is 850°C, for 3-12 hours in an air atmosphere or in an inert gas atmosphere or in a vacuum atmosphere (primary sintering step); forming a sintered body having a compact structure by heating subsequently to the heating in the preceding step in a temperature range of 900°C-1150°C for 0.5 - 8 hours under atmospheric pressure or under pressure in the same atmosphere as the preceding step (secondary sintering step); cooling the sintered body after the secondary sintering step; and conducting tertiary sintering by reheating in a temperature range of 900°C -1150°C under pressure in an inert gas atmosphere or in a vacuum atmosphere.