PURPOSE:To provide an electroless solder plating bath which forms a plating film at a high plating speed and has a dense plating film and a long life of a plating liquid by dissolving tin borofluoride, lead borofluoride, borofluoric acid, thiourea and a reducing agent in a specific amt. of water. CONSTITUTION:A soln. contg. 0.05-0.2mol/l tin borofluoride such as stannous borofluoride, 0.01-0.1mol/l lead borofluoride, 0.5-3mol/l thiourea, and <4mol/l borofluoric acid and contg. a proper amt. of sodium hypophosphite, hydrazine, EDTA, etc. as a reducing agent and if necessary, a surface active agent, etc. is prepd. as an electroless solder plating bath. If such electroless solder plating liquid is used, a dense solder plating layer of an alloy consisting of 85% Sn and 15% Pb is electroplated at a high speed. Since the concn. of the compsn. of said liquid is high, the using life is longer than the life of the conventional plating liquid.