PURPOSE: To determine the circuit constant of a band-pass filter in a short time with simple operation by providing a main body part composed of an electric circuit constituted having the same constitution with or constitution equivalent to a sound insulating device, an adjustment part which switches frequency band adjusting elements through a switch, and an electroacoustic transducer.

CONSTITUTION: A measuring instrument is carried to a site where the sound insulating device is expected to be used and the electroacoustic transducer (microcphone 1, headphones 6, etc.) for inputting and outputting a sound signal is mounted. Then while noises at the site are listened to through the headphones 6, a combination of frequency adjusting elements 7 and 8 which provide optimum frequency characteristics is determined by operating switches A and B and also replacing the headphones 6. Thus, a user or measuring person takes measurements and makes the decision while confirming results with the ear through the headphones, so a frequency band required for the site is selected freely, so less errors in measurement are made and the operation is easy, so that the necessary measurement time is shortened greatly.