PURPOSE: To make guide number variable through changing over of a switch by forming a charging voltage lower limit setting circuit which varies the lighting voltage of a neon tube and a discharging circuit which cooperates with this and in which the voltage slightly higher than the lighting voltage of said neon tube acts.

CONSTITUTION: Two resistance arrays R1, R2 which are associatedly operated by a rotary switch RSW are connected in parallel with a charging part 10 and a main capacitor C. Neon tubes NE1, NE2 are respectively connected to the two traveling contacts of the rotary switch RSW. When the main capacitor C is charged to the specified voltage, the neon tube NE1 goes on and charging is stopped. The resistance array R2 sets the upper limit value of the charging voltage. When the charging voltage exceeds the specified voltage and the neon tube NE1 goes on and in case the over-voltage is above the value determined by setting of the resistance array R2, the the neon tube NE2 goes on and a transistor Q0 goes ON. The excess energy is then discharged throught the resistance r3 and the charging voltage of the main capacitor C is brought within the specified value.