A machine learning device includes a first acquisition unit, a second acquisition unit, and a learning unit. The first acquisition unit acquires environmental information on a target space. The second acquisition unit acquires number-of-people information indicating a number of people in the target space. The learning unit learns the environmental information acquired by the first acquisition unit and the number-of-people information acquired by the second acquisition unit in association with each other. The environmental information includes an actual carbon dioxide concentration in the target space.