The invention provides a method of producing acrylic acid. The solid molding resin may include powdered molding resin or sheet-type molding resin. In case an error is occurring while writing a current information data section into a page of a current one of said flash memory devices, said current information data section is written into a non-flash memory. The temperature of the active element of the exhaust gas sensor is thus controlled stably at a desired temperature. In particular, the measuring result furnished by the level measuring device can be compared with an exact contents level value every time the contents level reaches the level monitored by a limit sensor whereby the level indication of the limit sensor changes its status. An auxiliary gas flow of inert gas is provided via forth port located between the third port and the atomic spectroscopic detector.