A first electrode (133) has an exposure portion (133b) exposed to a second measuring chamber (160), and a connection portion (133d) which is disposed at a position not exposed to the second measuring chamber (160) and is connected to the first lead (137) and which is a portion of the first electrode (133) located most distant from the second measuring chamber (160). The entire connection portion (133d) is located in a region A1 which extends from the second measuring chamber (160) over a distance of 1.0 mm or less.