The sensor includes a working electrode having a first wire with a first flat surface and an electrochemical element on the first flat surface. A reference electrode includes a second wire with a second flat surface and a counter electrode includes a third wire with a third flat surface. The first wire is a first sensor wire for the working electrode, the second wire is a second sensor wire for the reference electrode and the third wire is a third sensor wire for the counter electrode in a plurality of sensor wires. The second flat surface and the third flat surface face toward each other.
