To solve the problem that, in a conventional method of sampling a core for a strength test of an improvement pile, the outer peripheral surface of an inserted polyvinyl chloride pipe and improved soil are turned to a bonded state and the situation that pulling cannot be performed is caused.

In a core sampling method for inserting a cylindrical body 1 for core sampling to an uncured improvement pile and performing sampling, the cylindrical body 1 for core sampling, formed by covering the outer peripheral surface of a pipe 2 having an internal hole equivalent to the outer shape of a core 17 to be sampled with a synthetic resin tube 3 and tightly sealing and forming a housing layer housing a gas 15, water or the water and the gas in a part formed with the pipe outer peripheral surface on the inner side, is used, the cylindrical body for core sampling is inserted to the uncured improvement pile, and the pile is pulled out after the improved soil of the improvement pile is cured to some extent.