Fiber reinforced plastic (FRP) materials composed of glass/unsaturated polyester (GF/UP) are used in many fields because of their good properties. In this paper, the fiber reinforced plastic honeycomb sandwich construction made of GF/UP composites was investigated to examine its applicability as structural parts for underground use by utilizing its high flexural rigidity and high corrosion resistance. The creep tests were carried out by dipping the pamples in alkali buffer solution simulating the condition of under-ground water. The creep properties of FRP honeycomb sandwich construction were examined in flexural mode. The fracture appeared in the flexural or shear mode pattern depending on the loading conditions. The experimental results clarified the creep properties of the material and provided an important information about creep design for the application of FRP honeycomb sandwich structure in underground water.