The High Energy Accelerator Research Organization (KEK) plays a key role in particle physics experiments, as well as supporting the related research communities in Japanese universities. In order to ensure these critical missions, KEK has two large-scale computer systems: the Supercomputer System (KEKSC) and the Central Computer System (KEKCC).

The KEKSC is used mainly for collaborative research studies in theoretical elementary particle and nuclear physics and condensed matter physics, as well as for accelerator simulations. The system is composed of two different systems: Hitachi SR16000 model M1 (System A) and IBM Blue Gene/Q (System B).

The KEKCC caters for the research demands of particle physics, nuclear physics, the photon factory, neutron science, accelerator development, theory computation, etc. In addition this system provides an information infrastructure environment including Web, e-mail and Grid (EMI and iRODS) services and supports the research activities and collaborations of KEK.

As mentioned above the EMI Grid middleware is deployed in the KEKCC for analysing and sharing experimental data over the distributed systems. The system is operated under the Worldwide LHC Computing Grid (WLCG) project. The researchers working on the Belle~II, T2K, ILC and Kagra experiments perform their data analysis using the Grid infrastructure to manage large amounts of experimental data.

We would like to share our experiences and challenges related to the security, operation and experiment-specific applications, as well as the requirements for storage and computing resources, of the current KEKCC during the nearly four years of its operation. We focus in particular on the Grid Computing System. In addition we discuss prospects for the next KEKCC system, which will be newly introduced in September 2016.