Abstract: There is provided an electrochemical sensor 10, including: a plate-like member 11 having a first surface and a second surface that are in a front-back relationship and a sample liquid supply channel 11a arranged on a first surface side; a liquid reservoir 15, which is arranged on a second surface side of the plate-like member 11 and into which part of the sample liquid flows, the sample liquid having passed through a downstream end 11c of the plate-like member 11 that includes the supply channel; and a liquid sampling port 15a serving as an inlet of the liquid reservoir 15, which is spaced upstream of the supply channel 11 from the downstream end 11c of the plate-like member 11 in a plan view.