A terahertz wave detecting device includes a wavelength filter and a detection part. The wavelength filter is configured to convert terahertz waves to heat. The detection part is configured to detect the heat converted by the wavelength filter. The wavelength filter includes a wavelength selection layer and a terahertz wave absorption layer. The wavelength selection layer is configured to transmit terahertz waves of a prescribed wavelength among the terahertz waves. The terahertz wave absorption layer is provided in contact with the wavelength selection layer and the detection part, and contains a material for absorbing the terahertz waves of the prescribed wavelength.