The present invention provides a three-dimensional printer which determines a horizontal state of a bed. The three-dimensional printer of the present invention comprises: a printer main body which includes the bed and supports the bed; a molding unit which molds a molding product on the bed; a horizontal control unit for controlling horizontality of the bed; a horizontal detection unit including a housing main body mounted on the bed and including a liquid receiving unit which receives a light reflective liquid, mirror units which are supported to the housing main body and disposed in parallel to each other with an interval between the mirror units and the light reflective liquid received in the liquid receiving unit, a detected light projection unit which projects a detection light between the light reflective liquid and the mirror units, and a detected light receiving unit which is disposed oppositely to the detected light projection unit to receive the detected light; and a control unit which determines a horizontal state of the bed based on a light receiving signal from the detected light receiving unit.
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