1. A method of monitoring a machine condition, the method comprising: modeling a normal signal model performed by detecting a signal for monitoring a condition of a normal machine and converting the detected signal to data of the normal signal model in a time domain using a hidden Markov model (HMM) algorithm; calculating a probability value data of the monitoring signal at a subject machine performed by detecting a signal for monitoring a condition of the subject machine in real-time and converting the detected signal to the probability value data relative to the normal signal model using the HMM algorithm; determining a section having a deficiency where the probability value of the monitoring signal at the subject machine is not maintained constantly relative to the normal signal model; and updating the normal signal model to include the probability value data of a section determined to have the deficiency when the deficiency does not occurred at the section.