To provide a flux composition for soldering and a soldering paste, which can improve the joint strength of a electronic component to be mounted without causing foaming or cracks in slag of flux.

The flux composition for soldering contains a rosin derivative (A) obtained by adding and reacting a vinyl (thio) ether compound to a ring-opening half esterified compound of rosin alcohol and a compound having one or more cyclic acid anhydride radicals in one molecule. Further, the soldering paste is composed by mixing the flux composition and solder powder.