A simple method to monitor bunch length was tested in the damping ring. It measures spectrum of the beam signal from a pick-up BPM. Ratio of X-band component and C-band component was measured with diodes following band-pass-filters of the two frequencies. In order to calibrate this monitor, bunch length was controlled with the accelerating voltage of the DR. The resolution was estimated to be 30 fsec for typical bunch length of the equilibrium condition of the DR.