A curable composition includes: 100 parts by mass of a silicon-containing polymer having a Mw of 3,000 to 100,000 obtainable by hydrolysis-condensation of an organosilane mixture including R1SiX3, R2SiX3, R3R4SiX2 and R5SiX3, the total of R2SiX3 and R3R4SiX2 being 5 to 60 mol %, optionally a prepolymer, optionally a cyclic siloxane compound, 0.0001 to 10 parts by mass of an organic peroxide and optionally a metal catalyst, and 10 to 1,500 parts by mass of a filler, wherein R1 is a C2-6 alkenyl group, R2 is a C1-6 alkyl group, R3 and R4 are each a C1-6 alkyl group, R5 is a phenyl group optionally substituted with a C1-6 alkyl group, and X is a C1-6 alkoxy group, one or more of R2 to R4 is a methyl group, f represents a number of 2 to 10, g represents a number of 0 to 8, and n represents 1 or 2.