An occupant-left-unattended warning device includes: a transmitter/receiver to transmit an electromagnetic wave in an interior, and receive a reception signal; an estimating unit to estimate whether an occupant in the interior is a warning-necessitating occupant using a first method, assess whether it is possible to confirm that the occupant is a warning-necessitating occupant; a supplementary assessing unit to supplementarily assess whether the occupant is a warning-necessitating occupant using a second method in a case where it is not possible to confirm that the occupant is a warning-necessitating occupant; a warning necessity/unnecessity assessing unit to assess whether an occupant-left-unattended warning is necessary or unnecessary on a basis of the assessment or supplementary assessment result; and a control unit to control an output device to give a warning in a case where it is assessed that the occupant-left-unattended warning is necessary.
