To allow use of a synthetic resin mold by reinforcing the bottom part of a mold with an external mechanism while holding the mold, for preventing overflow deformation caused at mold release.

A means for holding the periphery of a specimen mold a such as concrete by one end of a plurality of clamp levers 16 born radially on a supporting board 1, a deformation preventing means comprising a seat plate 12 which prevents overflow deformation of the bottom part while tightly contacting to the bottom part of the mold a, and a means, comprising an air supply nozzle 13 at a central part where a plurality of clamp levers 16 face each other, for injecting a pressurized air between the bottom part of the mold a and the specimen b, are provided. With this configuration, no various auxiliary part is used nor such special form as is resistive to deformation is required as with non split-mold type synthetic resin cylindrical mold, or no work is required with the mold, for use in simplest state.