A 1‐cm‐long hollow parylene (C8H8) cylinder‐type target was irradiated with a long pulse CO2laser (400 J/50 ns) to produce a confined soft x‐ray laser source. The amplification of the Balmer‐alpha line (182.2 A˚) of the H‐like carbon was observed using two different gain determination methods. The time‐ and space‐integrated gain coefficient up to 2.2(+0.6/−1.1)cm−1was obtained from the ratio of the axial to transverse line intensity. By using the C8H7Cl target, the gain coefficient increased up to 2.8(+0.6/−1.1)cm−1, inferring the radiative cooling effect. The experimental results agreed with the simulation results within a factor of 5. By reducing the cylinder mass by a factor of 20, the simulation result gave the long gain duration up to 20 ns in FWHM and large gain region up to 1.3 mm in radius.