A valve actuator comprising: a housing; a motor disposed in an inner space of the housing; and a gear assembly disposed in the inner space of the housing and transmitting a driving force of the motor to a ball valve, wherein the gear assembly includes: an input gear coupled to a rotation shaft of the motor and rotating together with the rotation shaft; an output gear positioned apart from the input gear in a horizontal direction, coupled to a stem of the ball valve, and transmitting a rotational force of the input gear to the stem; and a power transmission gear (130, 140) transmitting the rotational force of the input gear to the output gear, wherein the power transmission gear (130, 140) includes a first gear set positioned between the input gear and the output gear, wherein the first gear set includes: a clutch shaft installed in the housing so as to be movable in a vertical direction; and a first gear and a second gear coupled to the clutch shaft and disengaged from each other or coupled to each other by vertical movement of the clutch shaft, characterized in that a coupling part for coupling the first gear to the second gear is protruded from a lower surface of the first gear coupled to the input gear, and a coupling part insertion groove into which the coupling part of the first gear is inserted is formed on an upper surface of the second gear, and wherein a push-up part is formed on the clutch shaft to push the first gear upward in contact with the coupling part of the first gear, and the push-up part is positioned in a push-up part insertion groove formed on a lower surface of the coupling part of the first gear. a housing; a motor disposed in an inner space of the housing; and a gear assembly disposed in the inner space of the housing and transmitting a driving force of the motor to a ball valve, wherein the gear assembly includes: an input gear coupled to a rotation shaft of the motor and rotating together with the rotation shaft; an output gear positioned apart from the input gear in a horizontal direction, coupled to a stem of the ball valve, and transmitting a rotational force of the input gear to the stem; and a power transmission gear (130, 140) transmitting the rotational force of the input gear to the output gear, wherein the power transmission gear (130, 140) includes a first gear set positioned between the input gear and the output gear, wherein the first gear set includes: a clutch shaft installed in the housing so as to be movable in a vertical direction; and a first gear and a second gear coupled to the clutch shaft and disengaged from each other or coupled to each other by vertical movement of the clutch shaft, characterized in that a coupling part for coupling the first gear to the second gear is protruded from a lower surface of the first gear coupled to the input gear, and a coupling part insertion groove into which the coupling part of the first gear is inserted is formed on an upper surface of the second gear, and wherein a push-up part is formed on the clutch shaft to push the first gear upward in contact with the coupling part of the first gear, and the push-up part is positioned in a push-up part insertion groove formed on a lower surface of the coupling part of the first gear. an input gear coupled to a rotation shaft of the motor and rotating together with the rotation shaft; an output gear positioned apart from the input gear in a horizontal direction, coupled to a stem of the ball valve, and transmitting a rotational force of the input gear to the stem; and a power transmission gear (130, 140) transmitting the rotational force of the input gear to the output gear, wherein the power transmission gear (130, 140) includes a first gear set positioned between the input gear and the output gear, wherein the first gear set includes: a clutch shaft installed in the housing so as to be movable in a vertical direction; and a first gear and a second gear coupled to the clutch shaft and disengaged from each other or coupled to each other by vertical movement of the clutch shaft, characterized in that a coupling part for coupling the first gear to the second gear is protruded from a lower surface of the first gear coupled to the input gear, and a coupling part insertion groove into which the coupling part of the first gear is inserted is formed on an upper surface of the second gear, and wherein a push-up part is formed on the clutch shaft to push the first gear upward in contact with the coupling part of the first gear, and the push-up part is positioned in a push-up part insertion groove formed on a lower surface of the coupling part of the first gear. a clutch shaft installed in the housing so as to be movable in a vertical direction; and a first gear and a second gear coupled to the clutch shaft and disengaged from each other or coupled to each other by vertical movement of the clutch shaft, characterized in that a coupling part for coupling the first gear to the second gear is protruded from a lower surface of the first gear coupled to the input gear, and a coupling part insertion groove into which the coupling part of the first gear is inserted is formed on an upper surface of the second gear, and wherein a push-up part is formed on the clutch shaft to push the first gear upward in contact with the coupling part of the first gear, and the push-up part is positioned in a push-up part insertion groove formed on a lower surface of the coupling part of the first gear.