A frequency doubler includes a voltage controlled oscillator outputting N (where, N is a natural number) signals having a first period and having different phases, and an XOR circuit receiving the N signals and outputting a signal having a second period that corresponds to a half of the first period, wherein the voltage controlled oscillator includes N nodes that correspond to the N signals and inverter units respectively connecting the N nodes, the N nodes are arranged so that, if a signal that starts from any one start node of the N nodes passes through the same number of the inverter units, it recurs to the corresponding start node, the XOR gate includes a first unit block set including N unit blocks that are connected to the same output node and match the N nodes in a one-to-one manner, and a second unit block set that is substantially the same as the first unit block set, wherein the first and second unit block sets share the output node.