To provide a preparation of a thermosetting resin composition enabling obtainment of a cured product having the excellent thermal resistance, preferable electric characteristics and the excellent film molding property.

The preparation of a thermosetting resin composition comprises the steps of obtaining a thermosetting resin by reacting a polyfunctional phenol compound represented by formula (1), a diamine compound represented by formula (2), and a component including an aldehyde compound, and obtaining a thermosetting resin composition by blending the thermosetting resin and an epoxy resin.