A cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus, said cartridge comprising: a rotatable member rotatable while carrying a developer on a surface thereof; and a coupling member (528; 628; 728; 828; 928; 1028; 1128; 1228; 1328; 1628; 2128; 2228; 4028) configured to rotate in a rotational direction during an image formation for transmitting a driving force to the rotatable member, said coupling member (528...) including (II-I) a driving force receiving portion (573a; 673a; 773a; 873a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 4073a) configured to receive the driving force for rotating said rotatable member, and (II-II) a supporting portion (573, 574; 673; 674; 773, 774; 873; 874; 973, 974; 1073, 1074; 1273, 1274; 1373, 1374; 1673, 1674; 2173; 4073, 4074) movably supporting said driving force receiving portion, wherein said supporting portion (573, 574...) includes (II-II-I) a projected portion (573; 673; 773; 873; 973; 1073; 1273; 1373; 1673, 2173; 2273; 4073) provided with said driving force receiving portion (573a...), and (II-II-II) an extending portion (574; 674; 774; 874; 974; 1074; 1274; 1374; 1674; 4074) extending in an extension direction, wherein a projecting direction in which said projected portion projects from said extending portion is crossing with the extension direction of said extending portion, characterized in that said extending portion (574...) extends at least in a circumferential direction of said coupling member and has an upstream end at its rear side with respect to the rotational direction and a downstream end at its leading side with respect to the rotational direction, wherein said extending portion (574...) extends from the rear upstream end (574a, 674a) toward said projected portion (573...) and toward the leading downstream end such that the leading downstream end of said extending portion is closer in radial direction to a rotational axis of said coupling member than the rear upstream end of said extending portion is to. a rotatable member rotatable while carrying a developer on a surface thereof; and a coupling member (528; 628; 728; 828; 928; 1028; 1128; 1228; 1328; 1628; 2128; 2228; 4028) configured to rotate in a rotational direction during an image formation for transmitting a driving force to the rotatable member, said coupling member (528...) including (II-I) a driving force receiving portion (573a; 673a; 773a; 873a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 4073a) configured to receive the driving force for rotating said rotatable member, and (II-II) a supporting portion (573, 574; 673; 674; 773, 774; 873; 874; 973, 974; 1073, 1074; 1273, 1274; 1373, 1374; 1673, 1674; 2173; 4073, 4074) movably supporting said driving force receiving portion, wherein said supporting portion (573, 574...) includes (II-II-I) a projected portion (573; 673; 773; 873; 973; 1073; 1273; 1373; 1673, 2173; 2273; 4073) provided with said driving force receiving portion (573a...), and (II-II-II) an extending portion (574; 674; 774; 874; 974; 1074; 1274; 1374; 1674; 4074) extending in an extension direction, wherein a projecting direction in which said projected portion projects from said extending portion is crossing with the extension direction of said extending portion, characterized in that said extending portion (574...) extends at least in a circumferential direction of said coupling member and has an upstream end at its rear side with respect to the rotational direction and a downstream end at its leading side with respect to the rotational direction, wherein said extending portion (574...) extends from the rear upstream end (574a, 674a) toward said projected portion (573...) and toward the leading downstream end such that the leading downstream end of said extending portion is closer in radial direction to a rotational axis of said coupling member than the rear upstream end of said extending portion is to. said extending portion (574...) extends at least in a circumferential direction of said coupling member and has an upstream end at its rear side with respect to the rotational direction and a downstream end at its leading side with respect to the rotational direction, wherein said extending portion (574...) extends from the rear upstream end (574a, 674a) toward said projected portion (573...) and toward the leading downstream end such that the leading downstream end of said extending portion is closer in radial direction to a rotational axis of said coupling member than the rear upstream end of said extending portion is to.