The effect of ambient-pressure reduction on multibubble sonochemiluminescence (MBSCL) is studied experimentally with a luminol solution through measurements of MBSCL intensity as a function of ultrasound irradiation time, applied voltage to a transducer and ultrasonic frequencies to accomplish high efficiency in chemical reactions. From the measurement of ambient-pressure dependence, it is shown that there is an ambient pressure that produces the maximum intensity of the MBSCL and the maximum intensity appears at higher ambient pressure as the applied voltage to the transducer increases. The highest intensity of MBSCL is obtained by appropriate reduction of ambient pressure both for various applied voltages and frequencies. This is caused by both the number of bubbles induced with supersaturation of the gas in a luminol solution and the variation in bubble dynamics.