The invention provides a method for producing a curing agent having an acidic substituent and an unsaturated maleimido group, including reacting, in an organic solvent, a maleimide compound (a) having at least two N-substituted maleimido groups in a molecule thereof with an amine compound (b) having an acidic substituent (represented by formula (I)); a thermosetting resin composition containing the curing agent (A) produced through the method and a compound (B) which is cured with the curing agent, wherein a cured product of the composition has a glass transition temperature of 200° C. or higher; and a prepreg and a laminated sheet produced therefrom. The curing agent produced through the method of the present invention has good solubility in an organic solvent and can provide a thermosetting resin composition exhibiting excellent adhesion-to-metal foil property, heat resistance, moisture resistance, flame retardancy, and copper cladding heat resistance, and low dielectric properties and low dielectric loss tangent. Therefore, the thermosetting resin composition can produce a prepreg or laminated sheet exhibiting excellent performance suitable for a printed wiring board for electronic devices and similar devices.