The present invention provides a water-retaining layer for retaining water in soil, the water-retaining layer being composed of a coagulated natural rubber latex； a greening system having the water-retaining layer provided； and a construction method of the greening system, comprising: an exposing step of removing surface soil in a construction target land and exposing underground soil, a spraying step of spraying a natural rubber latex on an exposed surface of soil, a coagulation step of coagulating the natural rubber latex to form a water-retaining layer, and a covering step of covering the water-retaining layer with the removed surface soil.
