We have developed a sensor element for detecting formaldehyde. The sensor element is made of porous glass impregnated with both beta-diketone and ammonium ions. We used three kinds of beta-diketone; acetylacetone, 1-phenyl-1,3-butanedione, and 1,3-diphenyl-1,3-propanedione. The three kinds of sensor element, which are initially colorless, turn yellow after exposure to formaldehyde, and absorption with a peak wavelength of 407-424nm appears. There is a linear relationship between the 407-424nm absorbance of the sensor element after exposure to formaldehyde, and the formaldehyde concentration. The sensor element also works cumulatively, and the absorbance changes of the three kinds of sensor elements are acetylacetone>1-phenyl-1,3-butanedione>1,3-diphenyl-1,3-propanedione, when exposed to the same concentration of formaldehyde in the atmosphere. We also found that both the formation and decomposition reactions of lutidine derivative (yellow dye) occur on the acetylacetone element at 20 degrees C, and the kinetic constant of the former is 100,000 times larger than that of the latter. On the other hand, only the formation reaction occurs on the 1-phenyl-1,3-butanedione element at 20 degrees C. Therefore, the acetylacetone element would be suitable for short-term measurements and the 1-phenyl-1,3-butanedione would be suitable for long-term measurements.