An electric heater apparatus for an electric vehicle comprising: a power supply part mounted in the electric vehicle; a switch control part for controlling a switching part according to a control command of an air-conditioner controlling part; the switching part switched according to a switching control signal of the switch control part for supplying electric power of the power supply part from a heating part; and the heating part for receiving the electric power of the switching part and generating heat, wherein the switching part includes a plurality of switching modules (13a, 13b, 13c) set to control an output load of the heating part, and wherein the switch control part operates one or more switching modules (13a, 13b, 13c) in serial order according to a target heating temperature; characterized in that the switching modules (13a, 13b, 13c) comprise IGBTs (Insulated Gate Bipolar mode Transistors) which are high-voltage switching elements, and wherein the switch control part divides a temperature control section of the heating part according to the number of the IGBTs, and wherein the switch control part carries out a duty control of one IGBT in the case that temperature control is carried out by electric current which does not exceed the maximum allowable current capacity of one IGBT, and carries out the duty control of one IGBT and controls other IGBTs to maintain a full-on operation state, in the case that the temperature control is carried out by electric current which is in an allowable current capacity range of two or more IGBTs; and wherein the switch control part turns off the IGBT which is in the duty control and carries out the duty control of one of the IGBTs which are in the full-on operation state so as to reduce the temperature, in the case that the IGBT which is in the duty control reaches the minimum duty control value of the IGBT during a temperature decrease control. a power supply part mounted in the electric vehicle; a switch control part for controlling a switching part according to a control command of an air-conditioner controlling part; the switching part switched according to a switching control signal of the switch control part for supplying electric power of the power supply part from a heating part; and the heating part for receiving the electric power of the switching part and generating heat, wherein the switching part includes a plurality of switching modules (13a, 13b, 13c) set to control an output load of the heating part, and wherein the switch control part operates one or more switching modules (13a, 13b, 13c) in serial order according to a target heating temperature; characterized in that the switching modules (13a, 13b, 13c) comprise IGBTs (Insulated Gate Bipolar mode Transistors) which are high-voltage switching elements, and wherein the switch control part divides a temperature control section of the heating part according to the number of the IGBTs, and wherein the switch control part carries out a duty control of one IGBT in the case that temperature control is carried out by electric current which does not exceed the maximum allowable current capacity of one IGBT, and carries out the duty control of one IGBT and controls other IGBTs to maintain a full-on operation state, in the case that the temperature control is carried out by electric current which is in an allowable current capacity range of two or more IGBTs; and wherein the switch control part turns off the IGBT which is in the duty control and carries out the duty control of one of the IGBTs which are in the full-on operation state so as to reduce the temperature, in the case that the IGBT which is in the duty control reaches the minimum duty control value of the IGBT during a temperature decrease control.