[en] Fuel blocks and removable reflector blocks, which constitute a core of VHTR, are supported by support posts. Each support post is in contact with a hot plenum block at the top end and with a lower plenum block at the bottom end through hemispherical seats to absorb a relative displacement generated by the lateral movement of both blocks by means of small inclination or rotation of support posts. Indicial response tests have been carried out by using a specified one-dimensional vibration model in order to estimate the effects of the support post length, the mass of hot plenum block and the hemispherical radii of both support and post seat on the vibrational characteristics in the support post structure. Futhermore the experimental results have been compared with the analytical ones obtained from the Lagrange's equation. The following are the conclusions derived. (1) The hemispherical radii of support post and post seat have a large effect on the frequency of vibration in the support post structure. (2) The frequency of vibration in the support post structure is predictable using the Lagrange's equation. (author)