By using the noncontact atomic force microscope with a frequency modulation detection method, the force gradient induced by the optical evanescent field was detected in a high vacuum. We succeeded in measuring the exponential distance dependence of the force gradient induced by the optical evanescent field. Furthermore, we investigated the incident beam intensity and bias voltage dependence of the force gradient induced by the optical evanescent field. We confirmed that the detection mechanism is not photothermal effect but the surface photovoltage effect.