  A microlensing event is mainly used to search for free-floating planets
(FFPs). To estimate the FFP mass and distance via the microlensing effect, a
microlensing parallax is one of the key parameters. A short duration of FFP
microlensing is difficult to yield a parallax by the observer's motion at a
recognisable level, so the FFP microlensing parallax is expected on the
simultaneous observation by multiple telescopes. Here, we approach the FFP
detection by considering a variation of FFP mass functions and the event rate
of accurately measured microlensing parallax. We used our FFP microlensing
simulator assuming a parallax observation between upcoming space-based missions
(Euclid and Roman) with full kinematics. As a result, we confirmed that the
event rate of accurately measured microlensing parallax (i.e. within a factor
of 2 uncertainty) does not simply follow the number of FFPs at a given mass but
the ratio of the FFP population per star. This is because the population ratio
determines the optical depth for a given mass and potential sources. In
addition, we found that the probability of the event that can estimate the FFP
mass and distance within a factor of 2 is not so high: approx. 40% of
Earth-mass, 16% of Neptune-mass, and 4% of Jupiter-mass FFP events under our
criteria. The probability can be improved by some technical approach such as
using high cadence and observation in parallax more than two observers.
