A method for producing a prepreg, comprising: preparing a reinforcing fiber sheet containing multiple reinforcing fiber bundles, a matrix resin composition, and first and second release sheets (2, 2'); forming a prepreg precursor by providing the matrix resin composition on the reinforcing fiber sheet; and sandwiching the prepreg precursor between the first and second release sheets (2, 2') so that first surfaces (2a, 2a') of the first and second release sheets (2, 2') make contact with the prepreg precursor and that the first and second release sheets (2, 2') respectively include extended portions (2A, 2A') that protrude outward from both edges of the prepreg precursor in a width direction; wherein the method further comprises: preparing a pair of elastic members, wherein the elastic members have a thickness prior to the compression process that is 1 to 10 times greater than a final thickness of a prepreg sheet; positioning the elastic members to face the extended portions of the second release sheet and to make contact with a second surface of the second release sheet to bring the extended portions (2A, 2A') of the first and second release sheets (2, 2') to abut each other; and compressing the prepreg precursor, the first and second release sheets (2, 2') and the elastic members all at once in a thickness direction of the prepreg precursor, wherein the reinforcing fiber sheet is made of unidirectional reinforcing fiber bundles, or wherein the reinforcing fiber sheet is made of short reinforcing fiber bundles two-dimensionally deposited at random, and a material of the elastic member has an elongation at break of 100% or greater and a Young's modulus of 0.01-40 MPa. preparing a reinforcing fiber sheet containing multiple reinforcing fiber bundles, a matrix resin composition, and first and second release sheets (2, 2'); forming a prepreg precursor by providing the matrix resin composition on the reinforcing fiber sheet; and sandwiching the prepreg precursor between the first and second release sheets (2, 2') so that first surfaces (2a, 2a') of the first and second release sheets (2, 2') make contact with the prepreg precursor and that the first and second release sheets (2, 2') respectively include extended portions (2A, 2A') that protrude outward from both edges of the prepreg precursor in a width direction; wherein the method further comprises: preparing a pair of elastic members, wherein the elastic members have a thickness prior to the compression process that is 1 to 10 times greater than a final thickness of a prepreg sheet; positioning the elastic members to face the extended portions of the second release sheet and to make contact with a second surface of the second release sheet to bring the extended portions (2A, 2A') of the first and second release sheets (2, 2') to abut each other; and compressing the prepreg precursor, the first and second release sheets (2, 2') and the elastic members all at once in a thickness direction of the prepreg precursor, wherein the reinforcing fiber sheet is made of unidirectional reinforcing fiber bundles, or wherein the reinforcing fiber sheet is made of short reinforcing fiber bundles two-dimensionally deposited at random, and a material of the elastic member has an elongation at break of 100% or greater and a Young's modulus of 0.01-40 MPa. sandwiching the prepreg precursor between the first and second release sheets (2, 2') so that first surfaces (2a, 2a') of the first and second release sheets (2, 2') make contact with the prepreg precursor and that the first and second release sheets (2, 2') respectively include extended portions (2A, 2A') that protrude outward from both edges of the prepreg precursor in a width direction; wherein the method further comprises: preparing a pair of elastic members, wherein the elastic members have a thickness prior to the compression process that is 1 to 10 times greater than a final thickness of a prepreg sheet; positioning the elastic members to face the extended portions of the second release sheet and to make contact with a second surface of the second release sheet to bring the extended portions (2A, 2A') of the first and second release sheets (2, 2') to abut each other; and compressing the prepreg precursor, the first and second release sheets (2, 2') and the elastic members all at once in a thickness direction of the prepreg precursor, preparing a pair of elastic members, wherein the elastic members have a thickness prior to the compression process that is 1 to 10 times greater than a final thickness of a prepreg sheet; positioning the elastic members to face the extended portions of the second release sheet and to make contact with a second surface of the second release sheet to bring the extended portions (2A, 2A') of the first and second release sheets (2, 2') to abut each other; and compressing the prepreg precursor, the first and second release sheets (2, 2') and the elastic members all at once in a thickness direction of the prepreg precursor, wherein the reinforcing fiber sheet is made of unidirectional reinforcing fiber bundles, or wherein the reinforcing fiber sheet is made of short reinforcing fiber bundles two-dimensionally deposited at random, and a material of the elastic member has an elongation at break of 100% or greater and a Young's modulus of 0.01-40 MPa.