We describe modification of various glasses by a CW Laser Backside Irradiation (CW-LBI) method. In this method, an absorbent which was attached on one side of a glass sheet, was irradiated from the other side through the glass with CW laser beam. Ar ion laser was used and copper foil was chosen as an absorbent. Silica, Pyrex and soda-lime glasses were tested as sample glasses. When the absorbent was irradiated, heated spot appeared in the glass adjacent to the absorbent and it ran to the other side of the glass in the path of laser beam. Then cylindrical modified zone that was transparent and crack-free was produced. Diameter of the modified column was 30~100 μm and the cylinder could grow up to 30 mm. The growth rate was approximately 200 mm/s. The dimension of modified zone depended on irradiation conditions, such as laser fluence or beam profiles. The threshold fluences for modification of the glasses were 1~2.5 × 105 W/cm2. Then the characteristics of modified zone were examined by etching in 5 wt. % HF solution. As a result, it turned out that modified zone consisted of two layers and etching rate of the inner part was lower than that of the outer part. The Raman spectroscopy revealed that the density of the inner part increased by the modification.