A method for fabricating a wiring board in which a target substrate having via holes and/or trenches is subjected to an electroless plating process while being immersed in an electroless plating solution to fill the via holes and/or the trenches with a plating metal, the method comprising the steps of: supplying the electroless plating solution to under the target substrate; diffusing an oxygen-containing gas into the electroless plating solution supplied under the target substrate; and allowing the electroless plating solution to overflow from over the target substrate, the diffusion of the oxygen-containing gas being temporarily suspended for a predetermined amount of time before the target substrate is completely immersed in the electroless plating solution, wherein the electroless plating solution includes at least a water-soluble metal salt, a reducing agent, and a complexing agent, and also includes a leveler comprised of at least one sulfur-containing organic compound represented by any one of the following general Formulae -: R 1 - n -R 2 R 1 - L 1 - n -R 2 R 1 -L 1 - n -L 2 -R 2 R 1 - n -L 3 R 1 -L 1 - n -L 3 where n is an integer equal to or greater than one; R 1 and R 2 independently represent an aliphatic or aromatic cyclic group including any arbitrary number of carbon atoms, oxygen atoms, phosphorus atoms, sulfur atoms, or nitrogen atoms, or a cyclic group in which one or more arbitrary substituents are bonded to the aliphatic or aromatic cyclic group; L 1 and L 2 independently represent a linear or branched one of a chain selected from the group consisting of an alkyl chain, an alkyl amino chain, an alkylene chain, and an alkoxy chain, and L 3 is selected from the group consisting of an alkyl group, an alkylene group, an amino group, an alkyl amino group, an alkylene amino group, a hydroxyl group, an alkyl hydroxyl group, an alkylene hydroxyl group, a carboxyl group, an alkyl carboxyl group, an alkylene carboxyl group, an alkyl amino carboxyl group, an alkylene amino carboxyl group, a nitro group, an alkyl nitro group, a nitrile group, an alkyl nitrile group, an amide group, an alkyl amide group, a carbonyl group, an alkyl carbonyl group, a sulfonic group, an alkyl sulfonic group, a phosphonic group, an alkyl phosphonic group, a sulfanyl group, a sulfinyl group, and a thiocarbonyl group. supplying the electroless plating solution to under the target substrate; diffusing an oxygen-containing gas into the electroless plating solution supplied under the target substrate; and allowing the electroless plating solution to overflow from over the target substrate, the diffusion of the oxygen-containing gas being temporarily suspended for a predetermined amount of time before the target substrate is completely immersed in the electroless plating solution, wherein the electroless plating solution includes at least a water-soluble metal salt, a reducing agent, and a complexing agent, and also includes a leveler comprised of at least one sulfur-containing organic compound represented by any one of the following general Formulae -: R 1 - n -R 2 R 1 - L 1 - n -R 2 R 1 -L 1 - n -L 2 -R 2 R 1 - n -L 3 R 1 -L 1 - n -L 3 where n is an integer equal to or greater than one; R 1 and R 2 independently represent an aliphatic or aromatic cyclic group including any arbitrary number of carbon atoms, oxygen atoms, phosphorus atoms, sulfur atoms, or nitrogen atoms, or a cyclic group in which one or more arbitrary substituents are bonded to the aliphatic or aromatic cyclic group; L 1 and L 2 independently represent a linear or branched one of a chain selected from the group consisting of an alkyl chain, an alkyl amino chain, an alkylene chain, and an alkoxy chain, and L 3 is selected from the group consisting of an alkyl group, an alkylene group, an amino group, an alkyl amino group, an alkylene amino group, a hydroxyl group, an alkyl hydroxyl group, an alkylene hydroxyl group, a carboxyl group, an alkyl carboxyl group, an alkylene carboxyl group, an alkyl amino carboxyl group, an alkylene amino carboxyl group, a nitro group, an alkyl nitro group, a nitrile group, an alkyl nitrile group, an amide group, an alkyl amide group, a carbonyl group, an alkyl carbonyl group, a sulfonic group, an alkyl sulfonic group, a phosphonic group, an alkyl phosphonic group, a sulfanyl group, a sulfinyl group, and a thiocarbonyl group. where n is an integer equal to or greater than one; R 1 and R 2 independently represent an aliphatic or aromatic cyclic group including any arbitrary number of carbon atoms, oxygen atoms, phosphorus atoms, sulfur atoms, or nitrogen atoms, or a cyclic group in which one or more arbitrary substituents are bonded to the aliphatic or aromatic cyclic group; L 1 and L 2 independently represent a linear or branched one of a chain selected from the group consisting of an alkyl chain, an alkyl amino chain, an alkylene chain, and an alkoxy chain, and L 3 is selected from the group consisting of an alkyl group, an alkylene group, an amino group, an alkyl amino group, an alkylene amino group, a hydroxyl group, an alkyl hydroxyl group, an alkylene hydroxyl group, a carboxyl group, an alkyl carboxyl group, an alkylene carboxyl group, an alkyl amino carboxyl group, an alkylene amino carboxyl group, a nitro group, an alkyl nitro group, a nitrile group, an alkyl nitrile group, an amide group, an alkyl amide group, a carbonyl group, an alkyl carbonyl group, a sulfonic group, an alkyl sulfonic group, a phosphonic group, an alkyl phosphonic group, a sulfanyl group, a sulfinyl group, and a thiocarbonyl group.