  Belle II is a major upgrade of the Belle experiment and operates at the
$B$-factory SuperKEKB in Japan. Since the SuperKEKB collider has a design
luminosity of 8$\;\times\;$10$^{35}$ cm$^{-2}$s$^{-1}$, about 40 times larger
than that of KEKB, Belle II aims to collect 50 ab$^{-1}$ of data over a period
of 8 years. The first data taking runs for physics analyses have started in
April 2018, with a lower luminosity than the designed one for commissioning
purposes. Even with the early dataset, having an integrated luminosity up to 20
fb$^{-1}$, Belle II can improve the current results in the Dark Sector field.
In this paper we will present the expected sensitivity of Belle II for
invisibly decaying Dark Photons, for Axion-Like Particles and for invisibly
decaying $Z^{\prime}$ assuming a $L_{\mu}-L_{\tau}$ model.
