A gear machining support device supports machining when a tooth of a gear is machined on a workpiece by relatively moving the workpiece and a machining tool while synchronizing rotations of the workpiece and the machining tool around respective center axis lines thereof. The gear machining support device includes: a target modification amount storage unit configured to store target modification amounts of at least two of modification elements of a tooth surface shape of the tooth of the gear, the modification elements including crowning, bias, a helix angle, a pressure angle, and a tooth profile roundness; and a correction amount determination unit configured to determine a correction amount of a machining control element during a machining operation such that the at least two of modification elements approximate the respective target modification amounts stored in the target modification amount storage unit.