An Sn--Zn lead-free solder alloy according to the present invention is constructed in a manner such that it is an Sn-based solder alloy indispensably containing 6 to 10 wt % zinc at least, and further containing 0.0015 to 0.1 wt % magnesium, said magnesium content being effective quantity for forming a protective magnesium oxide film on the solder surface and also for destroying said oxide film during soldering. When solder paste is preserved, the inside of solder particle is protected by the protective magnesium oxide film formed on the surface of solder particle, and a reaction between zinc and an activator is suppressed, so that preservation stability is improved, and at elevated temperature during soldering, a state where said protective oxide film is easily destroyed is obtained, so that good wettability is held.