A mandrel structure for use in manufacturing a fiber reinforced resin vessel by a filament winding process, includes: a shaft having a pair of axial ends configured to be supported by a drive unit; a mandrel having the shaft passed therethrough; and a pair of fittings having the shaft passed therethrough and projecting axially from respective axial ends of the mandrel. The mandrel is formed as a hollow shell made of a water-soluble material, and includes a pair of shaft holes receiving the shaft therein, and a receiving recess formed in a part of the mandrel surrounding each shaft hole and configured to engage the corresponding fitting in a rotationally fast manner.