The first measurement of He II (304 ¿) emission in the dayside magnetosphere has been done by the EUV scanner onboard Planet-B. A magnetic storm with the minimum Dst of -70 nT occurred during the observation. Before the initial phase of the magnetic storm, the average He+ density of ~1 cm-3 in the dayside magnetosphere was observed. During the main phase, the average He+ density rapidly increased up to 4--20 cm-3. During the recovery phase, the average density smoothly decreased to the lowest density, which is due to the daily ionospheric filling. The total loss of the ions to the outer magnetosphere is estimated by 3.6¿1030 ions. This is sufficiently large to populate the plasma sheet. ¿ 2001 American Geophysical Union