An aerosol generating device (1, 500) comprising: a battery (11, 510); a heater (13, 530) configured to heat an aerosol generating material; a voltage converter configured to output a preset fixed voltage by converting a voltage applied from the battery (11, 510) to a constant voltage; and a controller (12, 540) configured to determine whether the heater (13, 530) is operating in a preheating mode, and control electric power supplied from the voltage converter to the heater (13, 530) such that the preset fixed voltage is applied to the heater (13, 530) in the preheating mode, wherein the heater (13, 530) is further configured to operate in the preheating mode or a heating mode, and the controller (12, 540) is further configured to: control the electric power supplied to the heater (13, 530) such that a temperature of the heater (13, 530) reaches a preset preheating target temperature in the preheating mode, and control the electric power supplied to the heater (13, 530) such that the temperature of the heater (13, 530) is maintained below the preset preheating target temperature in the heating mode, and characterized in that the controller (12, 540) is configured to determine a period of time for preheating the heater (13, 530) in the preheating mode, based on an amount of power required for the temperature of the heater (13, 530) to reach the preset preheating target temperature and the preset fixed voltage applied to the heater (13, 530), and the period of time is relatively constant while the preset fixed voltage is applied to the heater (13, 530). a battery (11, 510); a heater (13, 530) configured to heat an aerosol generating material; a voltage converter configured to output a preset fixed voltage by converting a voltage applied from the battery (11, 510) to a constant voltage; and a controller (12, 540) configured to determine whether the heater (13, 530) is operating in a preheating mode, and control electric power supplied from the voltage converter to the heater (13, 530) such that the preset fixed voltage is applied to the heater (13, 530) in the preheating mode, wherein the heater (13, 530) is further configured to operate in the preheating mode or a heating mode, and the controller (12, 540) is further configured to: control the electric power supplied to the heater (13, 530) such that a temperature of the heater (13, 530) reaches a preset preheating target temperature in the preheating mode, and control the electric power supplied to the heater (13, 530) such that the temperature of the heater (13, 530) is maintained below the preset preheating target temperature in the heating mode, and characterized in that the controller (12, 540) is configured to determine a period of time for preheating the heater (13, 530) in the preheating mode, based on an amount of power required for the temperature of the heater (13, 530) to reach the preset preheating target temperature and the preset fixed voltage applied to the heater (13, 530), and the period of time is relatively constant while the preset fixed voltage is applied to the heater (13, 530). control the electric power supplied to the heater (13, 530) such that a temperature of the heater (13, 530) reaches a preset preheating target temperature in the preheating mode, and control the electric power supplied to the heater (13, 530) such that the temperature of the heater (13, 530) is maintained below the preset preheating target temperature in the heating mode, and characterized in that the controller (12, 540) is configured to determine a period of time for preheating the heater (13, 530) in the preheating mode, based on an amount of power required for the temperature of the heater (13, 530) to reach the preset preheating target temperature and the preset fixed voltage applied to the heater (13, 530), and the period of time is relatively constant while the preset fixed voltage is applied to the heater (13, 530).