A dew formation testing device has an adjustment tank capable of adjusting the temperature and humidity of air to predetermined temperature and humidity, a testing tank installed separately from the adjustment tank and having a sample base that has a mounting surface, onto which a testing sample W can be placed, and that is capable to cool the mounting surface, and ducts that link the adjustment unit and the testing tank. The testing tank is provided with an air guide member that, when air flowing into the testing tank through the duct flows onto the sample base from a side of the sample base, guides the air in the direction tilted downward at a predetermined angle, the guidance being performed at a position right above the sample base at an end thereof which is on the upstream side of the air flow.