Far-end measurement instrument for baseband frequency response, based on swept frequency method, is designed and constructed. In constituting the measurement instrument, it is important to stabilize modulation frequency characteristics of laser diode which is significantly affected by externally reflected light to the laser diode. It has been found that by coupling laser diode to a fiber with obliquely polished end face, the influence of externally reflected light can be negligibly small. Optimum coupling conditions, concerning tilt angle of fiber end and distance between fiber and laser facet are clarified. Moreover, automatic power control and temperature control circuit has been applied to the above laser to fiber coupler and far-end measurement instrument of baseband frequency response has been constructed. As a result, sufficient stabilization of modulation frequency characteristics to measure baseband frequency response of graded-index optical fiber has been attained.