To make a hidden hinge compact for installing a door in a frame body and collapsibly connecting the mutual doors such as a folding door.

This hidden hinge has a door side casing 3, a vertical frame side casing 4, first and second link materials 19 and 20 relatively rotatably connected by a pin 25 in a state of crossing respective one ends and having the respective other ends respectively horizontally rotatably pivoted to the casings 3 and 4, upper and lower respective guide arms 30 and 31 respectively arranged above and below these link materials 19 and 20, having holes 32 and 33 for inserting the pin 25 in a central part, having two arm parts extending in the horizontal direction from its central part, respectively arranging pin inserting holes 34 and 35 to be rotatably pivoted to the first and second link materials 19 and 20 in one arm part tip part and having guide pins 36 and 37 projected in the vertical direction in the other arm part tip part, and guide parts 3e and 4e respectively arranged inside the casings 3 and 4 and slidingly engaging with the guide pins 36 and 37 arranged in the respective guide arms 30 and 31.