To provide a vehicular headlamp capable of preventing a mounting failure of a light source, and informing a driver of a mounting failure; and a mounting failure detection system.

By bringing all of a contact terminal b1, a contact terminal b2 and a contact terminal b3 into contact with a contact terminal r1, a contact terminal r2 and a contact terminal r3, respectively, the contact terminal r1 is electrically connected to the contact terminal b3. Thereby, a conduction detection circuit 30 does not issue a warning. On the contrary, when any of the contact terminals b1, b2 and b3 is not brought into contact with corresponding one of the contact terminal r1, the contact terminal r2 and the contact terminal r3 due to a mounting failure of a bulb 10 or the like, the contact terminal r1 is not electrically connected to the contact terminal b3. Thereby, the conduction detection circuit 30 detects that the circuit is open, and informs a vehicle side of abnormality.