An onium salt represented by formula (a). Z-A-W—Y+(R)n X− (a) (In formula (a), Z, A, W, Y, (R)n, and X have the following meanings: Z represents a monovalent organic group having a ring structure provided with a conjugated π electron system, which may have one or more substituent groups; W represents a divalent organic group having a ring structure provided with a conjugated π electron system, which may have one or more substituent groups; A represents a divalent linking group containing a direct coupling of one or more bonds selected from a group consisting of a carbon-carbon single bond, a carbon-carbon double bond, and a carbon-carbon triple bond (any Z and/or W substituent group may form a ring structure in which one or more atoms included in Z and/or W are saturated or partially saturated together with A); Y is an iodine or sulfur atom, n=1 when Y is an iodine atom, and n=2 when Y is a sulfur atom; (R)n may be identical to or different from one another, and each (R)n is a monovalent organic group h
