A sensor for detecting material to be tested 100 having small variations in electrochemical measurement values includes a work electrode 1 and a counter electrode 2 integrated into one via an insulator 3. As a result of contact between the material to be tested and the work electrode 1, output voltage changes. The work electrode 1 smaller than the counter electrode 2 and the insulator 3 is installed on a part of the surface of the insulator 3, and a peripheral wall 4 for surrounding the work electrode 1 is formed on the insulator 3 to operate as a storage part.