A foreign matter detecting apparatus includes an oscillating unit, an optical system, a receiving unit, a scanning mechanism, and an operator. The oscillating unit generates a terahertz pulse wave and emits the terahertz pulse wave as irradiation light. The optical system guides the irradiation light to the first part of the container and condenses reflected light from the container. The receiving unit outputs a signal corresponding to the condensed reflected light and also measures an echo. The scanning mechanism scans a position of the irradiation light guided on the first part in a two-dimensional manner. The operator detects foreign matter in powder in a container based on at least one of a time waveform signal, a reflection image, a power spectrum, a tomographic image, and a frequency image. The time waveform signal is output from the receiving unit in chronological order.