A modified polycarbodiimide compound according to the present invention is obtained by modifying a polycarbodiimide compound derived from a diisocyanate compound with at least one aliphatic amine selected from the group consisting of diethylamine, methylisopropylamine, tert-butylethylamine, di-sec-butylamine, dicyclohexylamine, 2-methylpiperidine, and 2,6-dimethylpiperidine. A curing agent according to the present invention contains the modified polycarbodiimide compound. A thermosetting resin composition according to the present invention contains the curing agent, and a carboxyl group-containing resin having a carboxyl group in the molecules thereof or an epoxy resin having two or more epoxy groups per molecule thereof. As a result, it is possible to provide: a modified polycarbodiimidecompound that makes it possible to cure a resin composition at a relatively low temperature and that suppresses the curing of a resin composition at a drying step prior to a thermosetting step for theresin composition; a curing agent containing the modified polycarbodiimide compound; and a thermosetting resin composition containing the curing agent.