2006PASJ...58..335S

We propose a method to determine the distances of spiral arms in the galactic-center (GC) direction using radial-velocity gradients of approximately straight ridges in longitude-velocity (LV) diagrams of the CO and H <FONT SIZE="-2">I</FONT> line emissions. We show that the velocity gradient of an LV ridge is uniquely related to the distance. The arms and rings in the central region show large velocity gradients, while nearby arms appear as ridges with small velocity gradients. Applying this method, we determine the distances of the arms on the Sun-GC line, which has remained most difficult in the commonly used kinematical-distance method.