Provided is a biosensor that makes it possible to detect the electrical properties of a bio-related material contained in an analyte fluid such as an aqueous solution placed on a sensitive membrane and to observe the bio-related material at a high magnification with an observation device such as a microscope. The biosensor comprises: a substrate 10, a transparent semiconductor film 20 laminated on the substrate 10, a source electrode 30a and a drain electrode 30b formed at both ends of the transparent semiconductor film 20, an insulating film 40 laminated so as to cover the transparent semiconductor film 20 and the source electrode 30a and the drain electrodes 30b, a sensitive membrane 50 laminated on the insulating film 40, and a partition 60 that is formed at both ends of the sensitive membrane 50 to retain an analyte fluid 100, such as an aqueous solution or liquid culture containing a sample, on the sensitive membrane 50 and to retain the analyte fluid 100 in a specific region so that the bio-related material 200 can be placed on the sensitive membrane 50.