A short-bunched beam of electrons of an L-band linac of 38 MeV is guided to a cylindrical-closed resonator. Wavepackets of coherent transition radiation (CTR) generated from bunches passing through the mirrors are coherently summed up in the resonator. The amplification of the output of the resonator is confirmed in comparison with spontaneous CTR. The temporal structure of the output, observed with a hot-electron bolometer having the response time of 0.35 mus, shows oscillating characteristic of about I Vs in time scale. The result strongly suggests the inter-bunch distance varies rapidly over the duration of a macropulse, and it consequently results in degradation of the coherent summing up in the resonator. (C) 2004 Elsevier B.V. All rights reserved.