  We investigate the possibility of lens mass determination for a caustic
crossing microlensing event based on a space telescope observation. We
demonstrate that the parallax due to the orbital motion of a space telescope
causes a periodic fluctuation of the light curve, from which the lens distance
can be derived. Since the proper motion of the lens relative to the source is
also measurable for a caustic crossing event, one can find a full solution for
microlensing properties of the event, including the lens mass. To determine the
lens mass with sufficient accuracy, the light curve near the caustic crossing
should be observed within uncertainty of $\sim$ 1%. We argue that the Hubble
Space Telescope observation of the caustic crossing supplied with ground-based
observations of the full light curve will enable us to determine the mass of
MACHOs, which is crucial for understanding the nature of MACHOs.
