The invention aims to provide a thermosetting flux suitable for solder bonding of a semiconductor element and an electronic part and making solder bonding with a high bonding strength and a high heat resistant strength at a high temperature possible and a paste containing the flux and a non-lead type solder paste and with respect to the thermosetting flux, an epoxy resin, a hardening agent, and at least one of rosin derivatives having functional groups reactive on the epoxy resin and selected from maleic acid-modified rosin, a fumaric acid-modified rosin, and acrylic acid-modified rosin are used. The flux can be used in form of a solder paste while being mixed and kneaded with the non-lead type solder alloy powder.