To detect an air bubble of a specified length during a transfusion at a favorable precision by providing a first counter which always counts up when the output of an air bubble detecting means becomes a threshold value or lower, and a second counter which does not count during a period while a transfusion liquid flows, and counts during a period other than that.

An air bubble detecting section 103 transmits an ultra-sonic wave from a transmitting section 109, and the ultrasonic wave is received by a receiving section 110. A signal from the receiving section 110 is sampled based on a specified sampling cycle, and at the same time, the output signal is compared with a threshold value by a comparison circuit 115. When the output signal becomes the threshold value or lower, the countings of counting numbers are started by counters 111 and 112. The counter 112 synchronizes with a rotation detecting section, and performs the counting only during a period while a transfusion liquid does not flow, and also, the output signal from the receiving section 110 is at the threshold value or lower. Then, the counted numbers by the counters 111 and 112 are operated by an operation circuit 113, and whether or not the value is at the threshold value or higher is detected by an air bubble detecting circuit 114.