1 Introduction
1.1 Cloud-Cloud Collisions as a trigger of high-mass star formation
1.2 High-mass star forming region W33

2 Dataset
2.1 NANTEN2 12CO J=1-0 Observations
2.2 Nobeyama 45-m Telescope 12CO J=1-0, 13CO J=1-0, C18O J=1-0 Observations
2.3 Archive dataset

3 Results

4 Discussion
4.1 Gravitationally binding of the multiple velocity clouds
4.2 Expanding motion driven by feedback from high-mass stars
4.3 The cloud-cloud collision model
4.4 Cloud-cloud collision in W33
4.5 High-mass star formation in W33

5 Conclusions