An optical module having a flexible substrate having, even after actual manufacturing steps, excellent transmission characteristics of high-frequency signals and an advantage that electromagnetic field radiation is reduced even when it is connected with a package. The flexible substrate used in external connection of the package of the optical module uses a flexible substrate having a coplanar line to which a lead pin is fixedly attached, a grounded coplanar line which is in contact with the coplanar region, and a microstrip line which is in contact with the grounded coplanar line. The flexible substrate has an electrode layout in which an electromagnetic field component of a surface ground line and a signal line is more dominant than an electromagnetic field component of a back-surface ground line and the signal line in a region of the coplanar line adjacent to the grounded coplanar line.