A sulfonium borate complex that is capable of reducing the amount of fluorine ions generated during thermal cationic polymerization, and is capable of providing a thermal cationic polymerizable adhesive with low-temperature fast curing properties is represented by a structure represented by the formula (1). In the formula (1), R1 is an aralkyl group, R2 is a lower alkyl group, and R3 is a lower alkoxycarbonyl group. X is a halogen atom, and n is an integer of from 1 to 3.