Provided are: a thermal acid generator having a high acid generation temperature; and a resist composition using the same. The thermal acid generator is a sulfonic acid derivative compound represented by the following Formula (I): (wherein, R1 represents an aliphatic hydrocarbon group having 1 to 18 carbon atoms or the like, or a group represented by the following Formula (II): (wherein, Y1 represents a single bond or an alkanediyl group having 1 to 4 carbon atoms; R11 and R12 each independently represent an alkanediyl group having 2 to 6 carbon atoms, or the like; R13 represents a linear or branched alkyl group having 1 to 18 carbon atoms, or the like; a and b each represent 0 or 1; and either a or b is 1); R2 to R7 each independently represent a linear or branched alkyl group having 1 to 14 carbon atoms; the aliphatic hydrocarbon group or the like of R1 has no substituent or is substituted with a halogen atom or the like; and a methylene structure in the aliphatic hydrocarbon group or the like of R1 is optionally substituted with —O— or the like).