At Waseda University, various researches are done using a photocathode rf electron gunwith a 1.6 cell cavity. Now we are developing a new rf cavity specialized for producing anultra-short electron bunch, with the collaboration of High Energy Accelerator ResearchOrganization (KEK) . We have used SUPERFISH for designing the new rf cavity and PARMELAfor beam tracking. The new rf cavity has an extra cell following the 1.6 cell. The extracell can chirp the energy of electron bunch so we call it ECC (Energy Chirping Cell). ECCchirp the energy because we shortened the length of iris just before the ECC and also thelength of ECC itself. Moreover, electric field in ECC is made to be stronger than others.We have confirmed on PARMELA that ECC-rf-gun can generate an 100pC electron bunch less than100fsec with the energy of 4.5MeV at about 2.5m away from the cathode. Such an ultra-shortelectron bunch enables us to generate a coherent terahertz light using ultra-short electronbunch by synchrotron radiation or transition radiation. In this report, we would like tointroduce the detail of the design of this new ECC-rf-gun, the present progresses andfuture prospects.