In recent years, in consideration of demand of society, such as environmental protection and nature conservation, researches have been actively conducted on a fuel cell, which is an efficient and clean energy source. In particular, a polymer electrolyte fuel cell (PEFC) and a hydrogen internal combustion engine, which operate at low temperature, have high output, and are high in density, are expected to be used in homes or be mounted on vehicles. However, since these energy sources use hydrogen as fuel, they require a sensor for detecting leakage of hydrogen.
Conventionally, a sensor for detecting the concentration of a flammable gas such as hydrogen has a known structure in which an element case holding a sensor element is disposed on a circuit board, which is housed in a housing case (casing member) formed of resin (see Japanese Patent Application Laid-Open (kokai) No. 2008-267948 (FIG. 2)). In this sensor, the housing case has an annular flow channel forming portion which projects downward from the center of a lower portion of the housing case and opens downward. The sensor element is disposed to face the interior of the flow channel forming portion, and a hermetic seal is established between the element case and the housing case by means of an O-ring formed of rubber. Thus, an internal space (measurement chamber) into which an atmosphere to be detected is introduced is formed between the element case and the flow channel forming portion, and the concentration of a flammable gas is detected by the sensor element.