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We present the analysis of a binary microlensing event KMT-2016-BLG-2052, forwhich the lensing-induced brightening of the source star lasted for 2 seasons.We determine the lens mass from the combined measurements of the microlensparallax $\pie$ and angular Einstein radius $\thetae$. The measured massindicates that the lens is a binary composed of M dwarfs with masses of$M_1\sim 0.34~M_\odot$ and $M_2\sim 0.17~M_\odot$. The measured relativelens-source proper motion of $\mu\sim 3.9~{\rm mas}~{\rm yr}^{-1}$ is smallerthan $\sim 5~{\rm mas}~{\rm yr}^{-1}$ of typical Galactic lensing events, whilethe estimated angular Einstein radius of $\thetae\sim 1.2~{\rm mas}$ issubstantially greater than the typical value of $\sim 0.5~{\rm mas}$.Therefore, it turns out that the long time scale of the event is caused by thecombination of the slow $\mu$ and large $\thetae$ rather than the heavy mass ofthe lens. From the simulation of Galactic lensing events with very long timescales ($t_{\rm E}\gtrsim 100$ days), we find that the probabilities that longtime-scale events are produced by lenses with masses $\geq 1.0~M_\odot$ and$\geq 3.0~M_\odot$ are $\sim 19\%$ and 2.6\%, respectively, indicating thatevents produced by heavy lenses comprise a minor fraction of long time-scaleevents. The results indicate that it is essential to determine lens masses bymeasuring both $\pie$ and $\thetae$ in order to firmly identify heavy stellarremnants such as neutron stars and black holes.

KMT-2016-BLG-2052L: Microlensing Binary Composed of M Dwarfs Revealed from a Very Long Time-scale Event