A copper-tin alloy plating bath comprising an aqueous solution having a pH of 4.5 or less, the aqueous solution containing a water-soluble divalent copper compound, a water-soluble divalent tin compound, a sulfur-containing compound represented by formula: R-(CH 2) l -S-(CH 2) m -S-(CH 2) n -R, wherein R is H, OH, or SO 3 Na, and l, m, and n are each independently an integer of 0 to 3, a hydroxyl group-containing aromatic compound, a surfactant, and a leveler, the surfactant being at least one nonionic surfactant selected from the group consisting of polyoxyethylene alkylamines, polyoxyalkylene phenyl ethers, and polyoxyalkylene naphthyl ethers, the leveler being at least one aromatic ketone or aromatic aldehyde selected from the group consisting of benzalacetone, cinnamaldehyde, α-methylcinnamaldehyde, α-hexylcinnamaldehyde, α-amylcinnamaldehyde, cuminaldehyde, benzaldehyde, and anisaldehyde, the water-soluble divalent copper compound being present in an amount such that the amount of divalent copper ions is 1 to 60 g/L, the water-soluble divalent tin compound being present in an amount such that the amount of divalent tin ions is 5 to 40 g/L, the sulfur-containing compound being present in an amount of 5 to 500 g/L, the hydroxyl group-containing aromatic compound being present in an amount of 1 to 50 g/L, the surfactant being present in an amount of 0.1 to 40 g/L, the leveler being present in an amount of 0.01 to 10 g/L, and the surfactant:leveler ratio being 1:1 to 100:1. wherein R is H, OH, or SO 3 Na, and l, m, and n are each independently an integer of 0 to 3, a hydroxyl group-containing aromatic compound, a surfactant, and a leveler, the surfactant being at least one nonionic surfactant selected from the group consisting of polyoxyethylene alkylamines, polyoxyalkylene phenyl ethers, and polyoxyalkylene naphthyl ethers, the leveler being at least one aromatic ketone or aromatic aldehyde selected from the group consisting of benzalacetone, cinnamaldehyde, α-methylcinnamaldehyde, α-hexylcinnamaldehyde, α-amylcinnamaldehyde, cuminaldehyde, benzaldehyde, and anisaldehyde, the water-soluble divalent copper compound being present in an amount such that the amount of divalent copper ions is 1 to 60 g/L, the water-soluble divalent tin compound being present in an amount such that the amount of divalent tin ions is 5 to 40 g/L, the sulfur-containing compound being present in an amount of 5 to 500 g/L, the hydroxyl group-containing aromatic compound being present in an amount of 1 to 50 g/L, the surfactant being present in an amount of 0.1 to 40 g/L, the leveler being present in an amount of 0.01 to 10 g/L, and the surfactant:leveler ratio being 1:1 to 100:1. the surfactant being at least one nonionic surfactant selected from the group consisting of polyoxyethylene alkylamines, polyoxyalkylene phenyl ethers, and polyoxyalkylene naphthyl ethers, the leveler being at least one aromatic ketone or aromatic aldehyde selected from the group consisting of benzalacetone, cinnamaldehyde, α-methylcinnamaldehyde, α-hexylcinnamaldehyde, α-amylcinnamaldehyde, cuminaldehyde, benzaldehyde, and anisaldehyde, the water-soluble divalent copper compound being present in an amount such that the amount of divalent copper ions is 1 to 60 g/L, the water-soluble divalent tin compound being present in an amount such that the amount of divalent tin ions is 5 to 40 g/L, the sulfur-containing compound being present in an amount of 5 to 500 g/L, the hydroxyl group-containing aromatic compound being present in an amount of 1 to 50 g/L, the surfactant being present in an amount of 0.1 to 40 g/L, the leveler being present in an amount of 0.01 to 10 g/L, and the surfactant:leveler ratio being 1:1 to 100:1.