[en] In order to supply 99m Tc stably, we have designed a 35 MeV/35 kW compact S-band linac for gamma-ray source. For the beam tracking simulation in cylindrical coordinates of (r, θ, z), three kinds of codes were used. SUPERFISH and POISSON was used for electromagnetic field calculation, and GPT (General Particle Tracer) derived the result of beam tracking. This design uses Cannon's E37307 klystron which has 5 MW output but it was calculated with an assumption of 6.5 MW. The accelerating structure is a 2/3 mode traveling wave accelerating type. The system consists of two klystrons and two accelerating structures. As a result of simulation, the electron energy is 35 MeV, and the maximum output is about 50 kW. The output of 5.5 MW per one Klystron can achieve 35 kW/35 MeV. The entire system was optimized including the 100 Mo target design and yield calculation of 99 Mo.(author)