According to an aspect of an embodiment, a substrate for connecting circuit boards comprises: a substrate member having a first surface and a second surface facing each other and a first end and a second end facing each other; a first signal line formed on the first surface of the first end; a second signal line formed on the second surface of the second end; a third signal line connecting the first signal line with the second signal line; a first ground plane arranged on the first surface and surrounding the first signal line; and a portion of the second signal line formed over the first ground, the portion comprising narrower than an other portion of the second signal line.