The invention provides a component built-in circuit substrate and a method of producing the same. The component built-in circuit substrate includes a pair of electrodes (3a) for connection having constructed wiring patterns on the surface and a core layer (1) of the etching agent (4) formed between the pair of electrodes (3a); an electronic part (7) having a pair of terminals (7a) for connection;a tin welding portion (6aa) for attaching the electrode (3a) and the terminals for connection (7a); a sealing resin portion (6bb) for filling the interval between the lower surface of the electronic part (7) and the surface of the core layer (1) and covering the etching agent (4) and the tin welding portion (6aa). By such a structure, unfavorable condition in the internal wiring substrate of parts can be prevented.