An electronic component substrate (1-1) includes an insulating base (10) and a flexible circuit board (20) mounted on the insulating base (10). The flexible circuit board (20) is a synthetic resin film provided thereon with terminal patterns (29) and a conductor pattern (25) whose surface is slidingly contacted with a slider. The insulating base (10) is a synthetic resin molded piece. The flexible circuit board (20) is insert-molded to the insulating base (10). The electronic component substrate (1-1) is produced by preparing the flexible circuit board (20) and first and second mold members (41, 45) having a cavity (C1) with a shape that corresponds to the external shape of the electronic component substrate (1-1). Then, the flexible circuit board (20) is accommodated in the cavity (C1) between the first and second mold members (41,45), and the cavity (C1) is filled with a molten molding resin. After the molding resin has solidified, the first and second mold members (41, 45) are removed.