Flux for use in solder paste and solder paste

An object of the present invention is to provide a flux for use in a solder paste, which can suppress roughness during reflow of inter-metallic compounds and deformed bumps and deformed fillet accompanying therewith even when there is metal powder containing Ag exceeding a predetermined amount, such as Sn-Ag series and Sn-Ag-Cu series solder alloy powders, particularly for a metal powder mixture including Sn-3Ag-0.5Cu series solder alloy powder and a predetermined amount of Ag powder, and also provides a solder paste. The flux for use in a solder paste according to the present invention is characterized by comprising: (a) base resin, (b) a solvent, (c) an active agent, and (d) a thickener. The base resin (a) is formed of resin (a-1) having a softening point exceeding 130 DEG C and resin (a-2) having a softening point below 130 DEG C. The amount of the resin (a-1) having a softening point exceeding 130 DEG C is less than 5% by weight with respect to the total amount of the flux for use in a solder paste.