European Space Agency's Gaia mission has made the 2nd data release (Gaia DR2) on 25th April 2018. Gaia DR2 provides position, parallax and proper motions for more than one billion stars with unprecedented accuracy and radial velocity for about 7 million bright stars. In this talk, I will summarise our Galactic dynamics studies with Gaia DR2. First, I will show that for the first time Gaia DR2 has revealed the velocity structures of the Galactic stellar disk in the large range of radii, 5<Rgal<12 kpc. We found many diagonal ridges in the Rgal vs. Vrot map. We also demonstrate that these features can be explained by the transient and recurrent spiral arms. Second, I will talk about our search for dwarf galaxies in Fermi-LAT extended unassociated source fields, and discuss implications for annihilating dark matter. Finally, I will talk about the prospect of the next Gaia data releases and the future astrometry missions, including Small-JASMINE and Gaia NIR.