  Microlensing is one of the most powerful methods that can detect extrasolar
planets and a future space-based survey with a high monitoring frequency is
proposed to detect a large sample of Earth-mass planets. In this paper, we
examine the sensitivity of the future microlensing survey to Earth-mass planets
located in the habitable zone. For this, we estimate the fraction of Earth-mass
planets that will be located in the habitable zone of their parent stars by
carrying out detailed simulation of microlensing events based on standard
models of the physical and dynamic distributions and the mass function of
Galactic matter. From this investigation, we find that among the total
detectable Earth-mass planets from the survey, those located in the habitable
zone would comprise less than 1% even under a less-conservative definition of
the habitable zone. We find the main reason for the low sensitivity is that the
projected star-planet separation at which the microlensing planet detection
efficiency becomes maximum (lensing zone) is in most cases substantially larger
than the median value of the habitable zone. We find that the ratio of the
median radius of the habitable zone to the mean radius of the lensing zone is
roughly expressed as $d_{\rm HZ}/r_{\rm E}\sim 0.2(m/0.5 M_\odot)^{1/2}$.
