Contents and distributions of formaldehyde in the air above the ocean surface of the western North Pacific and the Indian Ocean were observed. The determination was done by the spectrophotometric method with coloring reagents of chromotropic acid and AHMT on board the research vessels during the cruises of Ry-75-05 (April to May 1975, the western North Pacific) and KH-76-1 (January to April 1976, the western North Pacific to the eastern Indian Ocean) conducted by the RV Ryofu-maru and the RV Hakuho-maru which belong respectively to the Japan Meteorological Agency and the Ocean Research Institute of the University of Tokyo. The formaldehyde content in the air above the western North Pacific Ocean and the Indian Ocean was in the range from less than 1 microgram to a few micrograms of CHO in a cubic meter of air, or from less than 0.8 ppb to a few ppb by volume except in the vicinity of continents and big islands. This content in the maritime air is equal to the smaller values observed so far in rural areas on land, and it may be regarded as a natural background level of formaldehyde in the air in the lower layer of the earth's atmosphere.