This paper describes a method to measure the distortion level of a loudspeaker and an amplifier using a LMS (least-mean square) adaptive filter. The acoustic apparatus checks the distortion level and the residual noise using a bandpass filter having a sharp cut-off frequency characteristic and a sinusoidal oscillator. The above bandpass filter, however, is expensive and has a long response characteristic. It is impossible to check each harmonic distortion. Also, to check the harmonic distortion of a loudspeaker in a room with ambient noise, the experiment room needs to be an anechoic chamber. On the other hand, the proposed method offers an easy way to measure the specified harmonic distortion level with a small amount of hardware. Moreover, this method is not affected by ambient noise which has no correlation with the test signal. The measurement can be carried out in a noisy environment. The system consists of a 32-bit DSP96002, 2ch of 16-bit analog to digital converter, and a 5th Chebyshev filter with a 50 kHz sampling frequency. The effective of the proposed method confirmed by experiments using a loudspeaker carried out in a room with ambient noise.