A gear coupling device and a gear coupling method, which reduce a vibration and noise caused at the time of engagement of gears (G1, G2), are provided. The gear coupling device transmits power through the engagement of the gears and uncouples the gears from each other by causing at least one (G1) of the gears (G1, G2) to move in an axial direction of the gear (G1). Rotation sensors (S1, S2) detect rotations of the gears (G1, G2). A determining section (10) determines, on the basis of signals detected by the rotation sensors (S1, S2), whether or not an end surface of a tooth of the axially moving gear (G1) lies between an end surface of a first tooth of the other gear (G2) and an end surface of a second tooth of the other gear (G2). The first and second teeth are adjacent to each other. A driving section (20) causes the gear to move in the axial direction on the basis of the determination result made by the determining section (10) and causes the gears (G1, G2) to be coupled to each other.