An illumination system comprising a switch and an illumination apparatus, the illumination apparatus comprising: a non-isolated power supply circuit which outputs a DC power obtained by rectifying an AC voltage inputted from an AC power source having a ground phase through a pair of power feed lines (Wa1, Wa2) and stepping up the rectified voltage, wherein one of the power feed lines (Wa1, Wa2) is grounded; a light source unit having semiconductor light emitting elements which are connected in series and are turned on by the DC power outputted from the power supply circuit; and an apparatus housing which is formed of a conductive material that is grounded through a ground path (Wa3, Wa4) and on which the power supply circuit and the light source unit are mounted, wherein the switch is located in the grounded power feed line, characterized by when the grounded power feed line among the power feed lines (Wa1, Wa2) is cut off from ground by the switch, there is formed a current loop in which a forward current (If) from the AC power source flows through the semiconductor light emitting elements, wherein the current loop includes the AC power source, the power supply circuit, the semiconductor light emitting elements, a stray capacitance (Ca) which is formed between the light source unit and the apparatus housing, the apparatus housing and the ground path (Wa3, Wa4); no capacitors are connected in parallel to the semiconductor light emitting elements for blocking a current flow from the AC power source through the semiconductor light emitting elements when only the grounded power feed line is cut off; and the number of semiconductor light emitting elements connected in series is set such that a maximum value of the forward current (If) in the current loop is less than a predetermined value. a non-isolated power supply circuit which outputs a DC power obtained by rectifying an AC voltage inputted from an AC power source having a ground phase through a pair of power feed lines (Wa1, Wa2) and stepping up the rectified voltage, wherein one of the power feed lines (Wa1, Wa2) is grounded; a light source unit having semiconductor light emitting elements which are connected in series and are turned on by the DC power outputted from the power supply circuit; and an apparatus housing which is formed of a conductive material that is grounded through a ground path (Wa3, Wa4) and on which the power supply circuit and the light source unit are mounted, wherein the switch is located in the grounded power feed line, characterized by when the grounded power feed line among the power feed lines (Wa1, Wa2) is cut off from ground by the switch, there is formed a current loop in which a forward current (If) from the AC power source flows through the semiconductor light emitting elements, wherein the current loop includes the AC power source, the power supply circuit, the semiconductor light emitting elements, a stray capacitance (Ca) which is formed between the light source unit and the apparatus housing, the apparatus housing and the ground path (Wa3, Wa4); no capacitors are connected in parallel to the semiconductor light emitting elements for blocking a current flow from the AC power source through the semiconductor light emitting elements when only the grounded power feed line is cut off; and the number of semiconductor light emitting elements connected in series is set such that a maximum value of the forward current (If) in the current loop is less than a predetermined value.