PURPOSE: To eliminate the wasteful consumption of recording paper, by a method wherein a system calculating the variation in the strength of light emitting output is provided and a flicker phenomenon is detected by this strength variation information to perform control of a wave form recording apparatus which is, in turn, operated only when the flicker phenomenon is detected.

CONSTITUTION: The discharge light emitting output of a fluorescent lamp 1 to be observed is received by a light detecting element 2 and, after amplified by a signal treating circuit 3, the amplified one is subjected to A/D conversion to be sent to an operation control apparatus 4 where the digital value of a peak value is written in the internal memory. In the circuit 4, the peak value of memory is divided into groups and the difference of the max. value and the min. value in each group is calculated and either one of differences is compared to a set value to decide the presence of flicker by the size thereof. Based on this result, a signal is sent to an operation result display apparatus 5 to display the total of the number of decision results. A wave form recording apparatus 6 records a wave form from the apparatus 3 only when flicker is present.