To provide a thick layer mat capable of being constructed by spraying and a slope greening method making use thereof for ensuring good adhesion between growing base material and bedrock.

A thick layer mat as a bag body 12 combining the approximately square front side 14 with the rear side 16 is equipped with a fastener 20 capable of opening and closing the upper half part of the front side 14 and the rear side 16 formed of a latticed net. In construction, the bag body 12 is mounted on the slope to fix, and after the fastener 20 is opened to fill the the bag body 12 with growing base material by spray injection, the fastener 20 is closed.