1. A method for refreshing a semiconductor device having a memory cell array that is divided in a plurality of blocks, the method comprising: a first step of placing the semiconductor device in an externally accessible first state; a second step of conducting refreshing during the first state for a block among the plurality of blocks other than a block to be externally accessed; a third step of placing the semiconductor device in an externally inaccessible second state; and a fourth step of conducting refreshing during the second state for at least one of the blocks among the plurality of blocks with a time difference provided with respect to refreshing for the remaining blocks.