Laser‐target interaction was studied to make an efficient soft‐x‐ray source with a table‐top laser system. A 1‐J, 20‐ns ruby laser was focused on various planar targets (Z=6 to 82). The laser power intensity on the targets was about 1011–1012W/cm2. Spectra (2–35 nm) and intensities of soft x‐ray radiation from target plasma were measured by using a 2‐m grazing incidence spectrometer and a x‐ray diode. Even at this low laser power intensity, high‐energy radiation of 0.5–3 keV was obtained. Spectra from low‐Zmaterials show strong line radiations over a weak continuum and those from high‐Zmaterials consist of a continuum predominantly. The targetZ‐number dependence of the intensity of 0.5–3‐keV radiation has a periodic structure and relates to atomic shell structures. In this experiment the maximum x‐ray conversion efficiency was obtained from a22Ti target.