In a magnetic disk controller equipped with a cache memory for disks, the controller in accordance with the present invention includes high order paths for data transfer between a high order channel apparatus and the cache memory through a certain one of a plurality of channel adaptors, low order paths for data transfer between a low order device, e.g. a magnetic disk device, and the cache memory through a certain one of a plurality of device adaptors and a path for data transfer betweeen the low order device and the channel cevice without passing through the cache memory by selecting empty device adaptor and channel adaptor by the switching operation of a switch. When an interrupt is generated from the device, data transfer can be made easily even when the high order path and the low order path are busy, and path utilization efficiency can be improved.