A storage system comprises multiple memory packages and a storage controller. The multiple memory packages respectively comprise multiple nonvolatile semiconductor memory devices for storing data, and a memory controller for controlling the reading/writing of data from/to these multiple semiconductor memory devices, and the storage controller receives an I/O command issued from a host computer, creates, on the basis of the received I/O command, a first level command for controlling the multiple memory packages, and sends this first level command to the multiple memory packages. The memory controllers of the multiple memory packages create a second level command for the multiple nonvolatile semiconductor memory devices inside its own memory package, and estimate the power to be consumed in its own memory package. In a case where the estimated power consumption exceeds a preconfigured permissible power, suspends the execution of the received second level command.