An autonomic nervous activity monitor capable of predicting occurrence of abnormal reaction depending on autonomic nervous activity and used for medical practice and the like is provided. A blood component collecting apparatus 100 carries out frequency analysis to a pattern of heartbeat which is acquired by an ECG 300 from a donor, and calculates a high frequency component value [msec.sup.2] defined as an indicator for parasympathetic activity and a ratio value of a low frequency component to the high frequency component defined as an indicator for sympathetic activity to detect a change in these values on a time-series base. When a judgment circuit 204 predicts that abnormal reaction may occur to a donor after blood collection operation is started, a controller 200 controls a pump 9 to decrease a blood drawing or blood returning speed, or, to stop blood drawing or blood returning operation.