The invention relates to a hinge device for a folding door. The hinge device for the folding door is used for connecting internal side ends of two door leaves, and thus the two door leaves can be oppositely unfolded to enter the closing state or folded to enter the opening state. The hinge device comprises two mounting bars and a connecting bar; a mounting groove which sleeves the internal side end of a corresponding door leaf is formed in the external side of each mounting bar; tooth parts which are engaged with each other are arranged in the internal sides of the two mounting bars, and a combining groove is formed in the internal side of each tooth part; the connecting bar comprises two separated shaft parts, and the two shaft parts are respectively arranged into the combining grooves in the two mounting bars by sleeving so as to integrally connect the two mounting bars through the connecting bar; when the two mounting bars are positioned at the first position, the two door leaves are in the closing state, and the connecting bar is wrapped with the two mounting bar; and when two mounting bars are at the second position, the two door leaves are in the opening state, and the tooth parts of the two mounting bars are positioned into the connecting bar.