Disclosed is a gear hinge device employed in a portable apparatus including first and second housings. The gear hinge device connects the first and second housings in such a manner such that the first and second housings can be rotated about first and second hinge axes, respectively. The gear hinge device includes: a first driving unit comprising a first driving cam gear that is rotated about the first hinge axis and a second driving unit comprising a second driving cam gear that is engaged with the first driving cam gear and rotated about the second hinge axis that is parallel to the first hinge axis and a flexible auxiliary gear that is arranged coaxially to the second driving cam gear.