A method of refreshing a semiconductor memory device includes performing a first refresh operation for memory cells included in a memory cell array, and determining whether a command other than a refresh command is applied to the semiconductor memory device in a refresh cycle of the first refresh operation. The method further includes continuing to perform the first refresh operation when a command other the refresh command is applied to the semiconductor memory device in one refresh cycle of the first refresh operation, and performing a second refresh operation when a command other than the refresh command is not applied to the semiconductor memory device in one refresh cycle of the first refresh operation. A refresh time of the second refresh operation is greater than a refresh time of the first refresh operation.