A relay-welding detection device for detecting welding in a relay inside a charger, the device comprising: a first analog-to-digital converter (ADC 1, 121) that measures a voltage of a relay input terminal (INPUT); a second analog-to-digital converter (ADC 2, 122) that measures a voltage of a relay output terminal (OUTPUT); and a central processing unit (CPU, 130) that determines whether welding occurs by comparing the voltage of the relay input terminal (INPUT) with the voltage of the relay output terminal (OUTPUT), wherein the central processing unit, when a charger corresponds to one of a charging standby state, a charging completion state, and an error state, determines that welding occurs when the voltage of the relay input terminal (INPUT) is same as the voltage of the relay output terminal (OUTPUT), and determines that welding does not occur when the voltage of the relay input terminal (INPUT) is different from the voltage of the relay output terminal (OUTPUT); wherein the relay-welding detection device is characterized by a voltage drop unit that converts the voltage of the relay input terminal (INPUT) and the voltage of the relay output terminal (OUTPUT) to match specifications of the first analog-to-digital converter and the second analog-to-digital converter, respectively, wherein the voltage drop unit comprises a first resistor connected in series to the relay input terminal, and a second resistor connected to the relay input terminal and a point to which a reference voltage ref is applied, or the voltage drop unit comprises the first resistor connected in series to the relay output terminal, and the second resistor connected to the relay output terminal and the point to which the reference voltage ref is applied. a first analog-to-digital converter (ADC 1, 121) that measures a voltage of a relay input terminal (INPUT); a second analog-to-digital converter (ADC 2, 122) that measures a voltage of a relay output terminal (OUTPUT); and a central processing unit (CPU, 130) that determines whether welding occurs by comparing the voltage of the relay input terminal (INPUT) with the voltage of the relay output terminal (OUTPUT), wherein the central processing unit, when a charger corresponds to one of a charging standby state, a charging completion state, and an error state, determines that welding occurs when the voltage of the relay input terminal (INPUT) is same as the voltage of the relay output terminal (OUTPUT), and determines that welding does not occur when the voltage of the relay input terminal (INPUT) is different from the voltage of the relay output terminal (OUTPUT); wherein the relay-welding detection device is characterized by a voltage drop unit that converts the voltage of the relay input terminal (INPUT) and the voltage of the relay output terminal (OUTPUT) to match specifications of the first analog-to-digital converter and the second analog-to-digital converter, respectively, wherein the voltage drop unit comprises a first resistor connected in series to the relay input terminal, and a second resistor connected to the relay input terminal and a point to which a reference voltage ref is applied, or the voltage drop unit comprises the first resistor connected in series to the relay output terminal, and the second resistor connected to the relay output terminal and the point to which the reference voltage ref is applied.