Disclosed is a seat elevating device including a support unit, a driving unit, and an elevating unit. The support unit is structured such that a first end is coupled to a body of a vehicle and a second end extends upward from the first end and is connected to a first end and a second end of a bar. The driving unit is coupled to an inner side surface of the support unit and equipped with a drive shaft on a side surface thereof. The elevating unit is connected to the drive shaft via a gear unit and is provided with a groove to receive the bar so that the bar slides along the groove. The elevating unit is slidable because it is engaged with the gear unit connected to the drive shaft via a rack gear that extends in parallel with a sliding movement direction of the elevating unit.