R and D toward X-band high-gradient acceleration at KEK

The research and development of X-band acceleration has been conducted at KEK since 1989. The activity proceeded as the KEK's main task force until the ITRP in 2004, aiming at the linear collider. Even after the ITRP chose the superconducting technology, KEK has continued the research of normal conducting X-band technology as one of the basic researches for high gradient acceleration at high frequency. In this report, some relevant achievements were reviewed such as the PPM klystron and the HOM-suppressed accelerating structure operated at 100 MV/m. Then, we discuss how we proceed this research in near future by taking into account the status of the linear collider research in the world. (author)