To detect moving targets such as cars and ships, the National Institute of Information and Communications Technology (NICT) developed an along-track interferometric SAR (AT-InSAR) system and introduced its function to NICT's airborne SAR in 2011. This paper reports the validation of ship detection and verification of its performance. First, the performance of AT-InSAR was verified and confirmed by observing a running boat equipped with a 50-cm square trihedral corner reflector and GPS in the range direction. Then, ship detection was validated by observing a running boat without the corner reflector in the range direction. Finally, ship detection was validated and the accuracy of its velocity was verified by observing a running boat without the corner reflector at an angle of 40° to the flight direction. The results verified that its velocity can be estimated using the AT-InSAR data.