We applied our millimeter-scale laser Doppler velocimeter (µ-LDV) as an absolute blood flow rate sensor in a thin tube (0.5 mm inner diameter) for precise control of the blood flow rate. We proposed a sequence of signal processing, frequency weighting modification, and peak detection, and succeeded in the detection of the beat signal induced by flow at the center of the tube. Flow rates of 20, 40, and 60% hematocrit [volume percentage of red blood cells (RBCs) in blood] were measured. The errors with respect to the hematocrit change were 14.2% for 58.9 µl/min, 5.1% for 235.6 µl/min, and 3.6% for 471.1 µl/min (corresponding maximum flow velocity of 80 mm/s), whereas the error ranges determined by laser Doppler flowmetry (LDF) calculation, which is generally used for blood flow measurement by the laser Doppler effect, were 89.2, 42.7, and 19.5%, respectively. The results show the potential application of µ-LDV as an integrable absolute blood flow rate sensor on a microchannel such as a thin tube and micro-total analysis system (µ-TAS) for a wide hematocrit range.