A method of setting revolutions per minute in real time of a rotating cutting tool for use with a machine tool, the method comprising: while a cutting process is performed on a workpiece by the rotating cutting tool, calculating a chatter threshold index (Th) value as a ratio of a vibration amplitude at a chatter frequency (M Fc) to a normal cutting vibration amplitude (M Ft), by Equation 1; Th = M Fc M Ft where MFc: vibration amplitude at chatter frequency, and MFt: normal vibration amplitude during the cutting process, whereby, in a case in which the chatter threshold index (Th) reaches a set value of chatter threshold index (Th), determining the case as a symptom of abnormal chatter vibration, calculating a chatter transient index (Tr) value as a ratio of an instantaneous gradient of the vibration amplitude (G i) at a specific timing (S1, S2, S3) and a moving average gradient of the vibration amplitude (G ma) of a past time interval (ma1, ma2, ma3) from the specific timing by Equation 2, Tr = G i G ma where G i: instantaneous gradient of the vibration amplitude, and G ma: moving average gradient of the vibration amplitude, whereby, when the chatter transient index (Tr) reaches a reference value of abnormal chatter vibration, it is judged that there is a symptom of abnormal chatter vibration, calculating recommended revolutions per minute (Ω j) of the tool by Equations 3 and 4, and adjusting the revolutions per minute of the rotating cutting tool to the recommended revolutions per minute of the rotating cutting tool in real time, Ω j = 60 ω n jN f where ω n: chatter frequency, j: integer representing stable revolutions per minute, and N f: number of blades of tool, κ = 60 ω n Ω ini N f where κ: revolutions per minute optimization degree representing a natural frequency and having an integer value, co ": chatter frequency, Ω ini: initial or current revolutions per minute, and N f: number of blades of tool, whereby the steps of calculating the recommended revolutions per minute (Q j) of the tool and adjusting the revolutions per minute of the rotating cutting tool to the recommended revolutions per minute (Ω j) are only performed when abnormal chatter vibration is detected and when at least one of the chatter threshold index (Th) and the chatter transient index (Tr) reaches its respective threshold value, and whereby, as K in Equation 4 is closer to j in Equation 3, the revolutions per minute of the rotating cutting tool is optimized. while a cutting process is performed on a workpiece by the rotating cutting tool, calculating a chatter threshold index (Th) value as a ratio of a vibration amplitude at a chatter frequency (M Fc) to a normal cutting vibration amplitude (M Ft), by Equation 1; Th = M Fc M Ft where MFc: vibration amplitude at chatter frequency, and MFt: normal vibration amplitude during the cutting process, whereby, in a case in which the chatter threshold index (Th) reaches a set value of chatter threshold index (Th), determining the case as a symptom of abnormal chatter vibration, MFc: vibration amplitude at chatter frequency, and MFt: normal vibration amplitude during the cutting process, calculating a chatter transient index (Tr) value as a ratio of an instantaneous gradient of the vibration amplitude (G i) at a specific timing (S1, S2, S3) and a moving average gradient of the vibration amplitude (G ma) of a past time interval (ma1, ma2, ma3) from the specific timing by Equation 2, Tr = G i G ma where G i: instantaneous gradient of the vibration amplitude, and G ma: moving average gradient of the vibration amplitude, whereby, when the chatter transient index (Tr) reaches a reference value of abnormal chatter vibration, it is judged that there is a symptom of abnormal chatter vibration, G i: instantaneous gradient of the vibration amplitude, and G ma: moving average gradient of the vibration amplitude, calculating recommended revolutions per minute (Ω j) of the tool by Equations 3 and 4, and adjusting the revolutions per minute of the rotating cutting tool to the recommended revolutions per minute of the rotating cutting tool in real time, Ω j = 60 ω n jN f where ω n: chatter frequency, j: integer representing stable revolutions per minute, and N f: number of blades of tool, κ = 60 ω n Ω ini N f where κ: revolutions per minute optimization degree representing a natural frequency and having an integer value, co ": chatter frequency, Ω ini: initial or current revolutions per minute, and N f: number of blades of tool, whereby the steps of calculating the recommended revolutions per minute (Q j) of the tool and adjusting the revolutions per minute of the rotating cutting tool to the recommended revolutions per minute (Ω j) are only performed when abnormal chatter vibration is detected and when at least one of the chatter threshold index (Th) and the chatter transient index (Tr) reaches its respective threshold value, and whereby, as K in Equation 4 is closer to j in Equation 3, the revolutions per minute of the rotating cutting tool is optimized. ω n: chatter frequency, j: integer representing stable revolutions per minute, and N f: number of blades of tool, κ = 60 ω n Ω ini N f κ: revolutions per minute optimization degree representing a natural frequency and having an integer value, co ": chatter frequency, Ω ini: initial or current revolutions per minute, and N f: number of blades of tool,