A test accelerator as coherent terahertz source (t-ACTS) project has been under development at Tohoku University, in which a generation of intense coherent terahertz (THz) radiation from sub-picosecond electron bunch will be demonstrated. We will supply wide-band coherent radiation from bending magnets in an isochronous accumulator ring and narrow-band coherent THz radiation using an undulator in a linac. Stable generation of very short electron bunch is one of the key issues in the t-ACTS project. The injector system is consists of a thermionic RF gun with two independent cavity cells, an alpha magnet and an accelerating structure. Velocity bunching in an accelerating structure is employed to produce the very short electron bunch. Most of components for the t-ACTS injector have already been installed, and we have started a high power RF processing of the gun cavities. Characteristics of electron bunch extracted from the RF gun are measured by varying phase and amplitude of input RF fields for the gun cavities.