We have developed a lightweight, compact, and highly sensitive Extreme ultraviolet (EUV) photometer for a space-borne instrument. The photometer is onboard Japan's Mars orbiter, Planet-B, which was successfully launched on July 4, 1998, and stayed in the parking orbits around the Earth for 6 months. The new photometer is designed for studying the spatial distribution of helium ions and atoms by detecting He II (304 ¿) and He I (584 ¿), which are major emission lines detectable around a planet. The photometer is a type of normal-incidence telescope that consists of a mirror, filters, and a detector. The mirror employs a new technology of a molybdenum/silicon multilayer coating and is designed to have peak reflectivity at 304 ¿. The photometer is capable of taking two-dimensional images by using the spin and orbital motions of the spacecraft. There are open questions concerning the Earth's plasmasphere and plasma sheet that cannot be resolved by in situ observations alone (e.g., global shape of the plasmasphere and cold ions in the plasma sheet). Our observations with the EUV photometer over a reasonable number of orbits will answer them. The photometer also shows an outstanding performance of measuring the Martian helium. ¿ 2001 American Geophysical Union