Cement concrete and mortar have some disadvantages such as low tensile strength, large drying shrinkage and low extensibility. Cracks frequently occur in various concrete structures and mortar walls. In particular, the large drying shrinkage is the most important factor affecting crack formation. Recently, surfactant-type shrinkage-reducing agents for concrete and mortar have been developed in Japan. The shrinkage-reducing agents (SRA) are not only added to fresh concrete and mortar, but also applied on hardened concrete and mortar surfaces. However, the effectiveness of SRA treatments for thin mortar walls in wooden buildings has not been reported till now. The purpose of this study was to make clear the effect of drying shrinkage reduction on the thin mortar by treating with SRA. Mortar specimens with or without SRA were prepared with thicknesses of 10, 20 and 40mm, and covered with an epoxy resin paint except for one side of each specimen. Then four types of SRA were treated at coverage rates of 100 and 300g/m2 on the surfaces of the mortar specimens without SRA, and the penetration depth of SRA was measured. The drying shrinkage of the mortar specimens was also measured up to 364 days of dry curing periods. The conclusions obtained from the test results are summarized as follows:(1) The drying shrinkage of the mortars is considerably reduced by treating with the shrinkage -reducing agents, and depends on the coverage rate of the agents and the thickness of the mortars. The drying shrinkage can be predicted as a function of the coverage rate.(2) The effectiveness of the shrinkage-reducing agent treatments increases with increasing coverage rate of the agents and with decreasing thickness of the mortars.(3) On the basis of the above results, the shrinkage-reducing agent treatments are applicable to reduce the drying shrinkage of the finishing mortars troweled on the metal lath bases of wooden buildings.