The present disclosure relates to a device and a method for measuring a thickness of an ultrathin film on a solid substrate. The thickness of the target ultrathin film is measured from the intensity of the fluorescence converted by the substrate and leaking and tunneling through the target ultrathin film at low detection angle. The fluorescence generated from the substrate has sufficient and stable high intensity, and therefore can provide fluorescence signal strong enough to make the measurement performed rapidly and precisely. The detection angle is small, and therefore the noise ratio is low, and efficiency of thickness measurement according to the method disclosed herein is high. The thickness measurement method can be applied into In-line product measurement without using standard sample, and therefore the thickness of the product can be measured rapidly and efficiently.