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A collision between two molecular clouds is one possible candidate forhigh-mass star formation. The HII region RCW 36, located in the Vela molecularridge, contains a young star cluster with two O-type stars. We present new COobservations of RCW 36 with NANTEN2, Mopra, and ASTE using $^{12}$CO($J$ = 1-0,2-1, 3-2) and $^{13}$CO($J$ = 2-1) line emissions. We have discovered twomolecular clouds lying at the velocities $V_\mathrm{LSR} \sim$5.5 and 9 kms$^{-1}$. Both clouds are likely to be physically associated with the starcluster, as verified by the good spatial correspondence among the two clouds,infrared filaments, and the star cluster. We also found a high intensity ratioof $\sim$0.6-1.2 for CO $J$ = 3-2 / 1-0 toward both clouds, indicating that thegas temperature has been increased due to heating by the O-type stars. Wepropose that the O-type stars in RCW 36 were formed by a collision between thetwo clouds, with a relative velocity separation of 5 km s$^{-1}$. Thecomplementary spatial distributions and the velocity separation of the twoclouds are in good agreement with observational signatures expected for O-typestar formation triggered by a cloud-cloud collision. We also found adisplacement between the complementary spatial distributions of the two clouds,which we estimate to be 0.3 pc assuming the collision angle to be 45$^{\circ}$relative to the line-of-sight. We estimate the collision timescale to be$\sim$10$^5$ yr. It is probable that the cluster age by Ellerbroek et al.(2013b) is dominated by the low-mass members which were not formed under thetriggering by cloud-cloud collision, and that the O-type stars in the center ofthe cluster are explained by the collisional triggering independently from thelow-mass star formation.

RCW 36 in the Vela Molecular Ridge: Evidence for a high-mass star cluster formation triggered by Cloud-Cloud Collision