Defects in a railhead and web can be detected with rail inspection cars and rail inspection tools equipped with special transducers such as wheel probes. However, bottom edges of rails, easily damaged regions due to contact to soil and fastening, are blind zones for the conventional ultrasonic inspection technique in which signals are input from a railhead. In this paper, inspection technique for defects in bottom edges of rails using guided waves were described. Dispersion curves for a rail obtained by a semi analytical finite element calculation show that a lot of propagating modes exist in a frequency region used for guided wave inspection, eg. 10kHz – 200kHz. Guided wave modes mainly vibrating a bottom edge of a rail were selected among the many modes in dispersion curves, and then defect detection tests were done using the selected modes. Good reflections from an artificial defect and a rail end were obtained with a vertically vibrating mode.