plex salt dye obtained by complexing the azo dye with copper and represented by the following formula (7): ##STR110## wherein A means a benzene or naphthalene ring having a sulfone group or a sodium, potassium or lithium salt thereof, Y and Z individually denote a group of the following formula (a): ##STR111## and Z2 denotes a group of the following formula (b): ##STR112## wherein R2 is a hydrogen