A highly miniaturized biosensor and a sensor unit, which can meet a demand for further miniaturization. With this invention, miniaturization is possible, and the number of production steps including those for assembling individual parts can be reduced. Accordingly, mass production will be possible, and cost reduction and high reliability will be achieved. A light emitting unit (21) and a light receiving unit (22) are disposed in a same recess (24) formed on a surface of a semiconductor substrate (23), and a light shielding cover substrate (27) having a first light guide section (25) and a second light guide section (26) is disposed on an upper side of the semiconductor substrate (23). Since the light receiving unit (22) is formed with arrayed light receiving elements (28), the same number of the second light guide sections (26) as the light receiving elements (28) are provided on the light shielding cover substrate (27), and the light receiving surfaces of the light receiving elements (28) are connected to one end of each of the second light guiding sections (26) so as to conform to each other without any gap.