A drum unit detachably mountable to a main assembly (100A; 4100A) of an electrophotographic image forming apparatus, the main assembly (100A; 4100A) including a driving shaft (101; 2210; 2301; 2410; 13101) provided with a recess (101a; 2210a; 2310a; 4101a; 13101a) on an outer peripheral side surface thereof, said drum unit comprising: a photosensitive drum; and a coupling member (328; 428; 628; 928; 1028; 1128; 1228; 1328; 1670; 2128; 2228; 2328; 2428; 2528; 2628) provided at said photosensitive drum, said coupling member (328; 428; 628; 928; 1028; 1128; 1228; 1328; 1670; 2128; 2228; 2328; 2428; 2528; 2628) including, (II-I) a driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a) configured to enter the recess (101a; 2210a; 2310a; 4101a; 13101a) and receive a driving force for rotating said photosensitive drum from the driving shaft (101; 2210; 2301; 2410; 13101), and (II-II) a supporting portion (372, 374; 473, 474; 673, 674; 973, 974; 1073, 1074; 1173, 1174; 1273, 1274; 1373, 1374; 1673, 1674; 2173; 2273; 2473, 2474; 2573, 2574; 2673, 2674) movably supporting said driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a), wherein at least a part of said driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a) where said driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a) is capable of receiving the driving force, is disposed inside said photosensitive drum. a photosensitive drum; and a coupling member (328; 428; 628; 928; 1028; 1128; 1228; 1328; 1670; 2128; 2228; 2328; 2428; 2528; 2628) provided at said photosensitive drum, said coupling member (328; 428; 628; 928; 1028; 1128; 1228; 1328; 1670; 2128; 2228; 2328; 2428; 2528; 2628) including, (II-I) a driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a) configured to enter the recess (101a; 2210a; 2310a; 4101a; 13101a) and receive a driving force for rotating said photosensitive drum from the driving shaft (101; 2210; 2301; 2410; 13101), and (II-II) a supporting portion (372, 374; 473, 474; 673, 674; 973, 974; 1073, 1074; 1173, 1174; 1273, 1274; 1373, 1374; 1673, 1674; 2173; 2273; 2473, 2474; 2573, 2574; 2673, 2674) movably supporting said driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a), (II-I) a driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a) configured to enter the recess (101a; 2210a; 2310a; 4101a; 13101a) and receive a driving force for rotating said photosensitive drum from the driving shaft (101; 2210; 2301; 2410; 13101), and (II-II) a supporting portion (372, 374; 473, 474; 673, 674; 973, 974; 1073, 1074; 1173, 1174; 1273, 1274; 1373, 1374; 1673, 1674; 2173; 2273; 2473, 2474; 2573, 2574; 2673, 2674) movably supporting said driving force receiving portion (373a; 473a, 673a; 973a; 1073a; 1173a; 1273a; 1373a; 1673a; 2173a; 2273a; 2373a; 2473a; 2573a; 2673a),