sites, which hold crucial clues about the birth processes of clusters of young stellar objects (YSOs) and massive stars. In this connection, the target of this paper is a massive star-forming site W33, which is located in the inner Galaxy. Previous studies have reported that W33 hosts several massive OB stars and star-forming clumps, and filaments [@westerhout58; @immer13; @immer14; @maud17; @kohno18 and references therein].

Based on the radio line and continuum observations [@downes80; @haschick83; @lockman89; @anderson14; @anderson15] and the VLT/SINFONI near-infrared (NIR) spectroscopic observations [@messineo15], several ionized clumps/H[ii]{} regions and massive OB stars have been investigated in the W33 complex [see Figure 1 in @kohno18]. In the W33 complex, @messineo15 reported that the population of massive stars (including O4-7 type stars) formed during $\sim$2–4 Myr ago. At least six dust continuum clumps at 870 $\mu$m were detected toward the W33 complex within an area of 15 pc $\times$ 15 pc [e.g., @contreras13], which were referred to as W33 Main, W33 A, W33 B, W33 Main1, W33 A1, and W33 B1 [see Figure 1 in @kohno18]. @kohno18 identified two CO bipolar outflows toward W33 Main and W33 A, and the dynamical timescales of those outflows were computed to be $\sim$3 $\times$ 