Surface reactions in Si(100) etching with HCl gas are studied using the surface photoabsorption (SPA) method and a quadrupole mass spectrometer. The adsorption of Cl atoms on the Si surface causes the intensity of the SPA signal to increase, when HCl gas is supplied. After HCl gas injection is stopped, the signal intensity decreases due to the etching reactions. By studying the time constants and the activation energies, two types of etching reactions are found. Below 1000 K, the etching products are SiCl4, and the activation energy for this etching process is estimated at about 22 kcal/mol. On the other hand, above 1000 K, mainly SiCl2 molecules are desorbed from the Si surface, and the activation energy is evaluated to be 59 kcal/mol.