Abstract: The invention provides an anesthesia monitor being able to directly make a separation and a determination in real time all components of a gas in which components having the same integer molecular weight exists, more particularly an expiration gas and an inspiration gas as the object of analysis. The monitor has a small size, a low price and a operational facility. A microscopic quantity of the expiration gas is introduced from a circular route of an anesthesia circuit for an inhalation anesthesia to the anesthesia monitor through a material pipe-line. A gas portion is transmitted by a rotary pump through valves and opening and closing valves to a measurement cell within a vacuum chamber arranged in a gap of between magnetic poles of permanent magnets in a Fourier transformation mass analyzer so that the expiration gas is subjected to a high speed sampling. An ionization of the ions is carried out within the measurement cell.