We seek to understand how the 4.8GHz formaldehyde absorption line is distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More specifically, we look for the relationship among the H_2_CO, ^12^CO, and ^13^CO spectral lines. The four regions of MON R2 (60'x90'), S156 (50'x70'), DR17/L906 (40'x60'), and M17/M18 (70'x80') were observed for H_2_CO (beam 10'), H110{alpha} recombination (beam 10'), 6 cm continuum (beam 10'), ^12^CO (beam 1'), and ^13^CO (beam 1'). We compared the H_2_CO, ^12^CO, ^13^CO, and continuum distributions, and also the spectra line parameters of H_2_CO, ^12^CO, and ^13^CO. Column densities of H_2_CO, ^13^CO, and H_2_ were also estimated. We found out that the H_2_CO distribution is similar to the ^12^CO and the ^13^CO distributions on a large scale. The correlation between the ^13^CO and the H_2_CO distributions is better than between the ^12^CO and H_2_CO distributions. The H_2_CO and the ^13^CO tracers systematically provide consistent views of the dense regions. Their maps have similar shapes, sizes, peak positions, and molecular spectra and present similar central velocities and line widths. Such good agreement indicates that the H_2_CO and the ^13^CO arise from similar regions.

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