The study was carried out to develop a 3D-printing support material which has water solubility, moisture resistance, high Tg, and high toughness. For this purpose, polyester resins having functional groups were prepared and their properties were determined. By controlling structure of the polymer, the polyester showed both solubility in neutral water at 70 °C and high moisture resistance. Furthermore, the Tg of the polymer was above 100 °C, which is higher than the chamber temperature of 3D printers using ABS, so the polymer is suitable as a support material in 3D printing of ABS. However, the polyester was too brittle for the production of tough, flexible filaments that are needed for 3D printing. Compounding the polyester with elastomer and compatibilizer was investigated to improve toughness of the polyester. Using a batch mixer / twin screw extruder, three components were mixed. The cross sections of the composites were observed by SEM. Submicron domains of dispersed elastomer were observed in the polymer matrix. The toughness of the composite was high enough to produce filaments with sufficient flexibility for 3D printing. Filaments of the composite were made by single screw extruder. 3D printing of model material (ABS) and the water-soluble support material was examined. The support material showed good solubility in neutral water at 70 °C after printing and it was easy to remove from ABS.