A programmable frequency divider, which is a core module of a frequency synthesizer using a Phase Locked Loop (PLL) for generating very high frequencies, includes a divided clock generator dividing a frequency of an input clock signal Fin by a first divide ratio (N+1) or a second divide ratio N according to a divide ratio control signal MC to generate a plurality of divided clock signals Dout; a counting unit counting the number CNT of the plurality of divided clock signals Dout, by performing swallow mode counting and program mode counting sequentially on the plurality of divided clock signal Dout; a control signal generator generating the divide ratio control signal MC, using the number CNT of the plurality of divided clock signal Dout, a count S by the swallow mode counting and a count P by the program mode counting, the count P corresponding to a maximum of the number CNT, feeding the divide ratio control signal MC back to the divided clock generator, and generating a reset control signal RST for resetting the counting unit.