A foreign-matter detecting apparatus for detecting a foreign matter in powder stored in a container , the container having a first part causing a most part of a terahertz pulse wave to transmit and a second part causing a terahertz pulse wave not to transmit but to reflect the terahertz pulse wave, the foreign-matter detecting apparatus comprising: an oscillating unit configured to generate a terahertz pulse wave and emit the terahertz pulse wave as irradiation light (L1); a telecentric optical system configured to guide the irradiation light (L1) emitted from the oscillating unit to the first part of the container and condense reflected light (L2) reflected from the container ; a receiving unit configured to output a signal corresponding to the reflected light (L2) condensed by the telecentric optical system ; a scanning mechanism configured to scan a position of the irradiation light (L1) guided by the telecentric optical system on the first part in a two-dimensional manner; and an operating unit configured to detect a foreign matter in the powder in the container based on at least one of a value corresponding to a time waveform signal output from the receiving unit in chronological order, a reflection image having respective pixel values found by time-integrating the time waveform signals, a power spectrum being found by calculating the time waveform signal by Fourier transform, and a frequency image having respective pixel values found by calculating the time waveform signal by Fourier transform, wherein the telecentric optical system includes a half mirror configured to reflect the irradiation light (L1) emitted from the oscillating unit and to transmit the reflected light (L2) reflected by the container , and wherein the scanning is performed by swinging the half mirror . an oscillating unit configured to generate a terahertz pulse wave and emit the terahertz pulse wave as irradiation light (L1); a telecentric optical system configured to guide the irradiation light (L1) emitted from the oscillating unit to the first part of the container and condense reflected light (L2) reflected from the container ; a receiving unit configured to output a signal corresponding to the reflected light (L2) condensed by the telecentric optical system ; a scanning mechanism configured to scan a position of the irradiation light (L1) guided by the telecentric optical system on the first part in a two-dimensional manner; and an operating unit configured to detect a foreign matter in the powder in the container based on at least one of a value corresponding to a time waveform signal output from the receiving unit in chronological order, a reflection image having respective pixel values found by time-integrating the time waveform signals, a power spectrum being found by calculating the time waveform signal by Fourier transform, and a frequency image having respective pixel values found by calculating the time waveform signal by Fourier transform, wherein the telecentric optical system includes a half mirror configured to reflect the irradiation light (L1) emitted from the oscillating unit and to transmit the reflected light (L2) reflected by the container , and wherein the scanning is performed by swinging the half mirror . <EOS>