Images of defects in a plate-like structure can be obtained from amplitude distributions measured using the scanning laser source technique. This paper discusses high-speed non-contact imaging for defects in a plate. Fundamental experiments conducted using a contact piezoelectric transducer as a receiving device revealed the effect of the generated wave shape and measurement speed on the defect images. Generating tone-burst waves consisting of multiple frequency components and averaging the images obtained in multiple frequency bands (frequency image averaging, FIA) gave clearer defect images in which noisy effects due to low signal-to-noise ratio and spurious images due to multiple reflections in high-speed imaging were reduced. Moreover, FIA was shown to be effective for high-speed non-contact imaging using a laser Doppler vibrometer.