A concentration degree determination device includes: an information acquisition unit configured to acquire a driving concentration degree of a driver of a vehicle； a reference setting unit configured to set a first reference or a second reference higher than the first reference for a manual driving mode that is switched from an automatic driving mode based on at least one of a first driving concentration degree and running information during execution of the automatic driving mode； a reference comparator configured to compare a second driving concentration degree during execution of the manual driving mode to the first or second reference； and an information output unit configured to output information according to a comparison result.
