A variable valve mechanism includes a low-speed cam, a high-speed cam, a low-speed rocker arm driven by the low-speed cam, a high-speed rocker arm driven by the high-speed cam, and a switching mechanism. The radiation detector system includes a radiation detector thermally coupled to a thermo electric cooler. Holding members are supported on sides of the side pipes opposite to sides of the same with which the brackets are in contact with the side pipes by pins, respectively. A first external access time is measured at the interface between a first read request and read data transmitted by the interface in response to the first read request.