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   51033
   We have demonstrated the capability of hard X-ray to reveal embedded
   protostars (class I objects). Then we next propose TO DETECT CLASS 0
   OBJECTS WHICH ARE YOUNGER THAN CLASS I OBJECTS. Our goal is; 1. to
   probe when a star begins to emit X-ray 2. to probe X-ray emission
   mechanism of protostars. Serpens dark cloud is the best target. In the
   cloud, there are many protostars, especially class 0 objects. Two of
   them, FIRS1 and a FU-Ori source, show extremely exotic features. FIRS1
   (class 0) has powerful jets, whose radio spectra imply synchrotron
   radiation from `relativistic' electrons. The FU-Ori source is the first
   detected protostar in ``FU Ori phase''.
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