A compact, dispersion-type mass spectrometer for analysis of gases of interest in the metabolism of plants has been developed. By means of a heated flexible inlet capillary 0.25mm in diameter, less than 15l/min of gas is taken from the plant-growing area using a differential pumping system (Fig. 1). Precise analysis of gases in high water-vapour pressure is performed under a constant capillary temperature of about 80°C. Ions produced by electron impact are separated according to their masses by a specially designed permanent magnetic field and simultaneously collected by independently adjustable collectors (Fig. 1). The magnetic field consists of a concave entrance field and a convex exit field. Properties of second-order aberrations are determined by the radii of the concave, and convex field, r and R, respectively and the angle of incidence, e, of the ion beam to the concave field (Fig. 2). These aberrations are minimum when r=31mm, R=50mm and e=24 degree, and three-dimensional focusing is applied clearly (Fig. 3). A mass difference such that the valley between two adjacent peaks is exactly zero is less than 1 within a mass range of 4 to 50 (Fig. 4). The advantages of this mass spectrometer in comparison with the well-known infra-red gas analyzer are:1) Such multiple gases as H2O, O2 and CO2 can be measured simultaneously.2) Several gases labeled by their stable isotopes, 13C, 15N and 18O, can be analyzed.3) The low gas consumption and short response time enables measurement of the changes in gas concentration in a small space.These characteristics have been clarified through the following experiments using this mass spectrometer:a) Distribution pattern of water vapour pressure on the surface of leaves of butter-head type lettuce and its response to sudden illumination were measured directly in the plant-growing area (Fig. 5 and 6).b) Simultaneous measurement of the photosynthesis and transpiration for a cut-leaf of butter-head type lettuce was made using the leaf chamber method (Fig. 7).c) Translocation of photosynthates from leaves to a green pepper pod using stable isotope 13CO2 was estimated by measuring the 13CO2 concentration emitted from the pod by respiration after 13CO2 feeding of the leaves (Fig. 8).