considered to supply interstellar matter to the CMZ and to sustain star formation therein [@Stark-2004]. Numerical simulations conducted by @Kim-2011 demonstrated that gas moving along $x_1$ orbits can fall down to $x_2$ orbits, forming a ring in which stars form.

Some observational data show that stars in the NSD also orbit in a manner somewhat consistent with the $x_2$ orbits. The first evidence of such rotation came from observations of maser emission by OH/IR stars and large-amplitude variables [@Lindqvist-1992; @Deguchi-2004]. The radial velocities of the massive clusters Arches and Quintuplet (approximately $+100~{{\mathrm{km~s}^{-1}}}$ at positive Galactic longitudes) also suggest prograde rotation in the NSD [@Figer-2002; @Liermann-2009]. However, @Stolte-2008 [@Stolte-2014] reported that the proper motions of Arches and Quintuplet clusters are too large for movement along closed $x_2$ orbits, but may be consistent with transitional trajectories from $x_1$ to $x_2$ orbits. To investigate the evolution of the NSD, we require the kinematics of stars of various ages; however, previously studied objects in the NSD are limited.

Here we report spectroscopic observations of classical Cepheids within $0.4^\circ$ of the Center. These stars are young ($\sim 25$ Myr), already suggesting that they belong to the NSD rather than to the extended bulge dominated by 