Abstract. This study aims to propose a way to estimate probability of scour occurrence of a railway bridge pier by means of ambient vibration monitoring. A remote real-time scour monitoring scheme utilizing ambient vibrations of piers is considered as an alternative method for conventional impact test. A stochastic approach to deal with uncertainty of estimated frequencies caused by relatively poor signal to noise of the ambient vibration is discussed. The probability distribution of the identified frequencies in a normal condition is modeled with a stable distribution, and the probability of scour occurrence is estimated using a logistic curve. The validity of proposed method is verified using monitoring vibration data during swollen river water period in the past.