The present invention provides a biaxially stretched sheet which is obtained by biaxially stretching an acrylonitrile-styrene copolymer, and wherein: the content of the acrylonitrile unit in the copolymer is 10-40% by mass; the copolymer has a weight average molecular weight of 100,000-250,000 and a polydispersity of 2.0-2.5; the area stretch ratio of the biaxially stretching of the biaxially stretched sheet is 4-10 times; the maximum orientation relaxation stresses in the MD direction and in the TD direction are 0.2-0.6 MPa, respectively; the absolute value of the difference between the maximum orientation relaxation stresses in the MD direction (a) and the maximum orientation relaxation stresses in the TD direction (b), namely |a-b| is 0.3 MPa or less; the sulfur content in the biaxially stretched sheet is 0 ppm or more but less than 100 ppm; and the content of volatile substances in the biaxially stretched sheet is 200-2,000 ppm.