A tag testing device uses an infrared camera to take an image of a plurality of radio frequency identification tags, compares the image with a stored standard tag pattern image storage, and detects defective tags based on the comparison. A radio wave transceiver bulk reads the tags using anti-collision, a tag response counter counts a number of tag responses, and a defective tag detector compares a number of heat emitting tags based upon the image processing with a number of tag responses counted by the tag response counter. If the number of heat emitting tags does not match a number of responsive tags, a number of the tags tested are changed by partially shielding the tags and the radio wave transceiver repeatedly bulk reads the tags using the anti-collision function while a shielding range is gradually changed, thereby narrowing down other possible defective but heat emitting tags.