The present invention relates to a fault diagnosis for a photovoltaic system. A fault diagnosis apparatus includes a temperature sensor unit measuring both an internal temperature value of the junction box and a surface temperature value of the solar module; a voltage sensor provided in the junction box; and an operation unit determining where a fault occurs among the solar module, an inside of the junction box, and the inverter by comparing measured values of the temperature sensor unit with a measured value of the voltage sensor. According to the present invention, initial investment cost is inexpensive. In addition, it is possible to detect solar module faults that are difficult to be checked by the naked eye.