represents a bond or an alkylene group, provided that when L1 is a bond, at least one of R12, R13, R14, R15 and R16 is --S--R18 or --SO--R18, R17 represents a hydrogen atom, an alkyl group or an acyl group, and Y1 represents a hydrogen atom or a substituted carbonyl group represented by the following formula: ##STR5## wherein R10 and R11 each has the same meaning as defined in formula (II);