An electrochemical gas sensor has a planar ceramic housing having a recess, a MEA, first and second electrically conductive gas diffusion membranes, and a metal lid fixed on the housing so as to cover the recess. The MEA is provided with an ionic conductive membrane, a first electrode on a surface of the membrane, and a second electrode on the opposite surface of the membrane. The first electrically conductive gas diffusion membrane is electrically connected to the first electrical connection. The second electrically conductive gas diffusion membrane is electrically connected to the second electrical connection. The lid presses the second electrically conductive gas diffusion membrane toward the MEA, and in the lid or in the bottom of the housing, a gas inlet is provided. The electrochemical gas sensor is easily made compact, small in the variations in the performances, and easily installed on a print circuit board.