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Recently, it has been discovered that fiber optic sensors are good alternative transducers for detecting several types of ultrasonic waves, including Lamb waves. However, in the case of fiber optic sensors, the sensitivity has directivity; that is, the sensitivity is variable according to the alignment direction of the sensor because the sensor dominantly measures the displacement induced by the change of gage length along the parallel direction to the sensor. Thus, considering the change of sensitivity with respect to the alignment direction of the sensor to an ultrasonic source is essential in order to detect the ultrasonic wave using a fiber optic sensor and to determine the absolute amount of the measured value. In this paper, the directivity of the interferometric fiber optic sensor to detect Lamb waves was investigated through both a theoretical analysis and an experimental one. The theoretical analysis showed that the sensitivity was related to the alignment angle of the sensor and to the ratio (L/λ) of the gage length of the sensor and the wavelength of the Lamb wave. These results were further verified in the experimental analysis.