A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2 m2. An infrared laser beam (λ0 =1047 nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2 MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2 mJ /pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300 eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.