A contactless thickness measuring apparatus is provided which includes an terahertz transmitter configured to receive the first optical path signal from the coupler and to generate a terahertz continuous wave using the first optical signal and an applied bias; an optical delay line configured to delay the second optical path signal output from the coupler; and an terahertz receiver configured to receive the terahertz continuous wave penetrating a sample and to detect an optical current using the terahertz continuous wave and the second optical path signal delayed. A thickness of the sample is a value corresponding to the optical current which phase value becomes a constant regardless of a plurality of measurement frequencies.