via conductor is filled with the plating film 54 a the plating film 54 a is simultaneously formed in the second opening 53 ao 1 and the first opening 51 a the plating film 54 a is simultaneously formed in the first opening 51 a the second opening 53 ao 1 and the third opening 53 ao 2 in this case the plating film 54 a is formed in the third opening 53 ao 2 a thickness sum t 7 fig2 f of a thickness of the electroless plating film 52 a and a thickness of the plating film 54 a on the second surface f 2 of the first resin insulating layer 50 a is 5 u03bcm or more in fig2 f the sum t 7 is 5 u03bcm an etching resist composition 55 u03b1 is applied on the plating resist 53 a and the plating film 54 a fig3 a in order to form the second via conductor 36 b and the second conductor layer 58 a from the plating film 54 a an etching resist 55 a is formed on the plating film 54 a fig3 b the etching resist 55 a is formed from the etching resist composition 55 u03b1 using a photographic technology a position at which the etching resist 55 a is formed is on the second via conductor 36 b by partially thinning the plating film 54 a the second via conductor 36 b and the second conductor layer 58 a are formed when the second.