A controller for a vehicular lamp (21-1 to 21-n; 51-1 to 51-n), comprising: a) power supply means (2, 32) for supplying power to a semiconductor light source (21-1 to 21-n; 51-1 to 51-n); and b) current detection means (3, 33) that includes a first resistor (Rs) connected serially to the power supply means (2, 32) and detects a light source driving current flowing through the semiconductor light source (21-1 to 21-n; 51-1 to 51-n), and b11) a second resistor (Rs') connected serially to the semiconductor light source (21-1 to 21-n; 51-1 to 51-n) via the first resistor (Rs), and a bypass section (4, 34) having a switch portion (10, 40) connected parallel to the second resistor (Rs'), characterised in that said current detection means further includes b12) a ground fault detection portion (ZD1, ZD10) adapted to detect a ground fault at a positive polarity output side or a negative polarity output side of the power supply means by monitoring the power supply voltage of the power supply means (2, 32) and to cause the switch portion (10, 40) to turn off when a ground fault occurs, b12') said ground fault detection portion (ZD1, ZD10) having a first state in which it causes said switch portion (10, 40) to be in the on state and turns-off said switch portion (10, 40) when it detects said ground fault, and a second state in which it causes said switch portion (10, 40) to remain in the off state; and b2) the controller for a vehicular lamp (21-1 to 21-n; 51-1 to 51-n) further comprises a changeover unit for carrying out changeover of the switch portion (10, 40) between on and off states by controlling said ground fault detection portion (ZD1, ZD10) into said first or second state. a) power supply means (2, 32) for supplying power to a semiconductor light source (21-1 to 21-n; 51-1 to 51-n); and b) current detection means (3, 33) that includes a first resistor (Rs) connected serially to the power supply means (2, 32) and detects a light source driving current flowing through the semiconductor light source (21-1 to 21-n; 51-1 to 51-n), and b11) a second resistor (Rs') connected serially to the semiconductor light source (21-1 to 21-n; 51-1 to 51-n) via the first resistor (Rs), and a bypass section (4, 34) having a switch portion (10, 40) connected parallel to the second resistor (Rs'), characterised in that said current detection means further includes b12) a ground fault detection portion (ZD1, ZD10) adapted to detect a ground fault at a positive polarity output side or a negative polarity output side of the power supply means by monitoring the power supply voltage of the power supply means (2, 32) and to cause the switch portion (10, 40) to turn off when a ground fault occurs, b12') said ground fault detection portion (ZD1, ZD10) having a first state in which it causes said switch portion (10, 40) to be in the on state and turns-off said switch portion (10, 40) when it detects said ground fault, and a second state in which it causes said switch portion (10, 40) to remain in the off state; and b2) the controller for a vehicular lamp (21-1 to 21-n; 51-1 to 51-n) further comprises a changeover unit for carrying out changeover of the switch portion (10, 40) between on and off states by controlling said ground fault detection portion (ZD1, ZD10) into said first or second state. b11) a second resistor (Rs') connected serially to the semiconductor light source (21-1 to 21-n; 51-1 to 51-n) via the first resistor (Rs), and a bypass section (4, 34) having a switch portion (10, 40) connected parallel to the second resistor (Rs'), b12) a ground fault detection portion (ZD1, ZD10) adapted to detect a ground fault at a positive polarity output side or a negative polarity output side of the power supply means by monitoring the power supply voltage of the power supply means (2, 32) and to cause the switch portion (10, 40) to turn off when a ground fault occurs, b12') said ground fault detection portion (ZD1, ZD10) having a first state in which it causes said switch portion (10, 40) to be in the on state and turns-off said switch portion (10, 40) when it detects said ground fault, and a second state in which it causes said switch portion (10, 40) to remain in the off state; and b2) the controller for a vehicular lamp (21-1 to 21-n; 51-1 to 51-n) further comprises a changeover unit for carrying out changeover of the switch portion (10, 40) between on and off states by controlling said ground fault detection portion (ZD1, ZD10) into said first or second state.