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    PR # 31281                                                                   
     Present ASCA limt of faint-source detection is determined by the back       
     ground statistics, and reached at 4.2x10E-14 erg cmE-2 sE-1 in 2-10 k       
     eV band at the ASCA deepest fields. The ASCA deep sky survey in the 2       
     -10 keV band has been one of the main scientific subjects since missi       
     on planning, and was expected to lead the large evidences of the CXB        
     origin. However, the present statistics of source number density in 2       
     -10 keV band at the limit cannot be developed by the present data.  I       
     t would be expected to be improved by the additional exposure time. W       
     e propose here the additional 400ks exposure at an AO-1 deep survey f       
     ield, Selected Area 57 in order to reach at the detection limit of 2x       
     10E-14 erg cmE-2 sE-1 in 2-10 keV band, and then to investigate the o       
    rigin of the hard emission of                                                
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