A back board attachment structure for a seat back (2, 2A) having a trim cover (5, 5A), a back board (6, 6A) and a first folding portion (5a1, 5Aa1) in which an edge portion (5a, 5Aa) on said back surface side of said trim cover (5, 5A) is folded back, a first fastener (7a, 7Aa) having a first element (7a2, 7Aa2), in which said first element (7a2, 7Aa2) is attached to said first folding portion (5a1, 5Aa1), a second folding portion (6a1, 6Aa1) in which an edge portion (6a, 6Aa) to be said backrest surface side of said back board (6, 6A) is folded back, and a second fastener (7b, 7Ab) having a second element (7b2, 7Ab2), in which said second element (7b2, 7Ab2) is attached to said second folding portion (6a1, 6Aa1), wherein said first fastener (7a, 7Aa) and said second fastener (7b, 7Ab) are made to be capable of engaging one another, and wherein - said back board (6, 6A) is configured by including a hard base material portion (6c, 6Ac) and a skin portion (6d, 6Ad) provided to cover one surface of said base material portion (6c, 6Ac), said skin portion (6d, 6Ad) has a skin extended portion (6d1, 6Ad1) that is extended by protruding from an edge portion (6a, 6Aa) of said base material portion (6c, 6Ac), and said second folding portion (6a1, 6Aa1) is formed on said skin extended portion (6d1, 6Ad1), or - said back board (6, 6A) has a soft portion that is softer than other portions, at an edge portion (6a, 6Aa) to be said backrest surface side, said second folding portion (6a1, 6Aa1) is formed by folding said soft portion, and said soft portion is a weak press portion having a less amount of compression than the other portions to become softer and thicker. a first folding portion (5a1, 5Aa1) in which an edge portion (5a, 5Aa) on said back surface side of said trim cover (5, 5A) is folded back, a first fastener (7a, 7Aa) having a first element (7a2, 7Aa2), in which said first element (7a2, 7Aa2) is attached to said first folding portion (5a1, 5Aa1), a second folding portion (6a1, 6Aa1) in which an edge portion (6a, 6Aa) to be said backrest surface side of said back board (6, 6A) is folded back, and a second fastener (7b, 7Ab) having a second element (7b2, 7Ab2), in which said second element (7b2, 7Ab2) is attached to said second folding portion (6a1, 6Aa1), wherein said first fastener (7a, 7Aa) and said second fastener (7b, 7Ab) are made to be capable of engaging one another, and wherein - said back board (6, 6A) is configured by including a hard base material portion (6c, 6Ac) and a skin portion (6d, 6Ad) provided to cover one surface of said base material portion (6c, 6Ac), said skin portion (6d, 6Ad) has a skin extended portion (6d1, 6Ad1) that is extended by protruding from an edge portion (6a, 6Aa) of said base material portion (6c, 6Ac), and said second folding portion (6a1, 6Aa1) is formed on said skin extended portion (6d1, 6Ad1), or - said back board (6, 6A) has a soft portion that is softer than other portions, at an edge portion (6a, 6Aa) to be said backrest surface side, said second folding portion (6a1, 6Aa1) is formed by folding said soft portion, and said soft portion is a weak press portion having a less amount of compression than the other portions to become softer and thicker. - said back board (6, 6A) is configured by including a hard base material portion (6c, 6Ac) and a skin portion (6d, 6Ad) provided to cover one surface of said base material portion (6c, 6Ac), said skin portion (6d, 6Ad) has a skin extended portion (6d1, 6Ad1) that is extended by protruding from an edge portion (6a, 6Aa) of said base material portion (6c, 6Ac), and said second folding portion (6a1, 6Aa1) is formed on said skin extended portion (6d1, 6Ad1), or - said back board (6, 6A) has a soft portion that is softer than other portions, at an edge portion (6a, 6Aa) to be said backrest surface side, said second folding portion (6a1, 6Aa1) is formed by folding said soft portion, and said soft portion is a weak press portion having a less amount of compression than the other portions to become softer and thicker.