  Analyzing the high-resolution CO-line survey of the Galactic plane with the
Nobeyama 45-m telescope (FUGIN), we show that the star-forming complex
G18.15-0.30+51 (G18) at radial velocity of 51 \kms is {a tight triple
association of a giant molecular cloud (GMC), HII regions, and a supernova
remnant (SNR).} The kinematical distance of G18 is determined to be $d=3.9\pm
0.2$ kpc for near solution for circular rotation, $12\pm 0.2$ kpc for far
solution, or $d=6.1\pm 0.1$, if it is in the 3-kpc expanding ring. The HI-line
absorption of radio continuum from the HII regions constrains the distance to
$5.6 \lesssim d_{\rm SNR}\le 7.6$ kpc. The $\Sigma-D$ (radio
brightness-diameter) relation yields the distance to the SNR of $d_{\rm
SNR}=10.1^{+11.5}_{-4.7}$ kpc, allowing for a minimum distance of 5.4 kpc. From
these we uniquely determined the distance of G18 to be $6.07\pm 0.13$ kpc in
the 3-kpc expanding ring with the SNR being physically associated. The
molecular mass of the GMC is estimated to be $M_{\rm mol}\sim 3\times 10^5
M_\odot$. The ratio of Virial to luminous molecular masses is greater than
unity in the central region and decreases outward to $\lesssim 0.2$ at the
cloud edge, indicating that the central region is dynamic, while the entire
cloud is stable. We discuss the origin of the G18 triple system and propose a
sustainable GMC model with continuous star formation.
