Pronunciation manner varies depending on the noise of the speech environment. This is known as the Lombard effect. It affects the acoustic features of speech such as intensity, pitch, duration, and spectral shape. Because vowels play an important role in Japanese speech recognition, special attention was paid to the variability of the acoustic features of vowels uttered in noise. First, the variability of the five Japanese vowels was examined using speech uttered while hearing noise through headphones. It was observed that the higher formants are unstable and that the energy in the middle frequency range of the vowel spectrum increases depending on the noise level. Based on these observations, a normalization method combining band limitation and spectral tilt compensation was proposed, and its effectiveness in the recognition of vowels was shown by experiment. Next, a new word recognition method utilizing this normalization technique was proposed. This method performs vowel normalization at the portion where both test and reference patterns are vowel‐like sounds. The experimental results show the significant effectiveness of the proposed method. The recognition rate obtained by the proposed method with normalization is 96% for 50 words uttered by two male speakers, while the recognition rate without normalization is 80%.