A method of manufacturing an interconnect substrate by electroless plating which causes a metal to be deposited without using a plating resist, the method including: (a) immersing a substrate in a catalyst solution including palladium, hydrogen peroxide, and hydrochloric acid to form a catalyst layer on the substrate; and (b) depositing a metal on the catalyst layer by immersing the substrate in an electroless plating solution to form a metal layer.