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   71166
   The cluster mass function and its evolutionary effect is the best
   measure to investigate the structure formation history of the universe.
   However, the observational study of the cluster evolution has so far
   been performed mainly in terms of the X-ray luminosities, which is
   indirect and subject to systematic errors. In order to obtain a tight
   constraint on the cosmological models, we are aiming to construct the
   cluster mass function in the medium redshift universe of z=0.2-0.3 with
   much less uncertainties. For this purpose, we compiled a flux-limited
   complete sample from the ROSAT All Sky Survey data, which consists of
   eight clusters. The ASCA observation of the three clusters proposed
   here will complete the mass measurements of our sample clusters.
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