A precise free-field calibration system of type LS2 microphones by the reciprocity technique is proposed. A source and a receiver microphone are each attached to a cylinder of 1/2 in. diameter, respectively, and the cylinders are set up perpendicularly at the center of the anechoic room. The output voltage of the receiver microphone and the input voltage of the source microphone are measured by an FFT analyzer, and the frequency transfer functions are averaged to improve the signal-to-noise ratio. To reduce the influence of the disturbing reflections in the anechoic room, the gating technique is adopted. Values for the position of the acoustic centers of MR112 microphones were measured by two methods---gating and conventional. The uncertainty of the values by the gating method was less than by the conventional method.