Provided is a self refresh oscillator which includes a plurality of inverters serially connected between an input terminal and an output terminal; a pull up driver for charging a first node in accordance with a level of the output terminal; a comparator for comparing a potential of the first node with a reference voltage and outputting the result to the input terminal; and a period adjusting unit for operating based on a level of the output terminal and adjusting an amount of current discharged into a ground of the first node in accordance with a temperature.