A group of contact pads (20EB) formed on a lower surface (20B) at a connection end of a board portion of an optical module board (20) includes, in order from the endmost end, a contact pad (20B1) conducting to a ground line (G), contact pads (20B2) and (20B3) conducting to transmission-side high-speed signal lines (S), a contact pad (20B4) conducting to the ground line (G), contact pads (20B5, 20B6, and 20B7) conducting to low-speed signal lines (S), a contact pad (20B8) conducting to the ground line (G), contact pads (20B9 and 20B10) conducting to receiving-side high-speed signal lines (S), and a contact pad (20B11) conducting to the ground line (G). Fixed terminal portions of a plurality of contact terminals (32ai) connected to the contact pads (20B2 and 20B3) conducting to the transmission-side high-speed signal lines (S) and the contact pads (20B9 and 20B10) conducting to the receiving-side high-speed signal lines (S) in a host connector (30) are electrically connected to conductive paths (16THL, 16RHL), which are continuous with high-speed signal lines formed on a plane shared with electrode portions (16BE2, 16BE3, 16BE9, and 16BE10) via the electrode portions (16BE2, 16BE3, 16BE9, and 16BE10) of a printed wiring board (16).