In a semiconductor relay system for supply a power source to a lamp load under the on/off control by a microcomputer using a semiconductor relay, a method of cutting off a circuit under an overcurrent, a first prescribed value is which is a current value higher than a rated load current for the load and a maximum current value supplied to the lamp load, and a second prescribed value is set which is a current value between said first prescribed value and said rated current value; the current value flowing through the lamp load is sampled at predetermined time intervals and comparing the current value thus sampled with the second prescribed value; and when a higher current than said first prescribed value flows through the lamp load, the current supplied to the lamp load is controlled within a prescribed range between said first prescribed value and said second prescribed value, and when the sampled current value higher that said second prescribed value is counted by a prescribed number of times, the sampled current is decided as an overcurrent thereby cutting off the semiconductor relay to stop the current supply to said lamp load.