A control method includes initially operating a compressor at the same RPM as an operating RPM of the compressor during a previous cycle, adjusting the RPM in consideration of an operation rate of the previous cycle such that the RPM is increased when the operation rate is greater than a first set value, reduced when the operation rate is less than a second set value, and maintained when the operation rate is within a range between the first set value and the second set value, and operating the compressor at the adjusted RPM, the first set value being greater than the second set value.