Dramatic traffic increase in modern day necessitated an extension of cross section of Tennozan tunnel, which was constructed by a side drift method 35 years ago, in a very difficult ground condition. Knowing that the initial construction had experienced large deformation and high pressure on concrete lining, a special care was taken for planning and selection of construction method. As a result of the actual extension of the tunnel cross section was completed only with minor deformation with fair confidence in tunnel stability. Displacement recorded for a ordinary section was less than 1mm at the depth of 2m and at most 3mm at surface. That recorded for an extended section, however, went up to 2-3 mm at the depth of 4-5m, and was 10-14mm at surface on the right side.