The modified polycarbodiimide compound of 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. The curing agent of the present invention comprises the modified polycarbodiimide compound of the present invention. The thermosetting resin composition of the present invention comprises a carboxyl group-containing resin having a carboxyl group in a molecule or an epoxy resin having two or more epoxy groups in one molecule, and the curing agent of the present invention. Thus, a modified polycarbodiimide compound that can allow a resin composition to be cured at a relatively low temperature and that can suppress curing of a resin composition in a drying step before a thermal curing step of a resin composition, a curing agent comprising the modified polycarbodiimide compound, and a thermosetting resin composition comprising the curing agent can be provided.