A blood flow measurement device , comprising: a light output device configured to output light of a preset wavelength band; a light receiving sensor configured to receive the light outputted from the light output device and reflected by a subject; and a processor , wherein the processor is configured to: control the light output device to irradiate light to a first position of the subject and a second position of the subject located in a first direction with respect to the first position, acquire intensities of the reflected light at the first position and the second position from the light receiving sensor that receives the reflected light reflected at the first position and the second position, determine a first value which is a difference between the intensities of the reflected light at the first position and the second position, control the light output device to irradiate light to a third position of the subject located in a second direction opposite to the first direction with respect to the second position, acquire an intensity of the reflected light at the third position from the light receiving sensor that receives the reflected light reflected at the third position, determine a second value which is a difference between the intensities of the reflected light at the second position and the third position, and determine a blood flow direction in the subject based on the first value and the second value. a light output device configured to output light of a preset wavelength band; a light receiving sensor configured to receive the light outputted from the light output device and reflected by a subject; and a processor , wherein the processor is configured to: control the light output device to irradiate light to a first position of the subject and a second position of the subject located in a first direction with respect to the first position, acquire intensities of the reflected light at the first position and the second position from the light receiving sensor that receives the reflected light reflected at the first position and the second position, determine a first value which is a difference between the intensities of the reflected light at the first position and the second position, control the light output device to irradiate light to a third position of the subject located in a second direction opposite to the first direction with respect to the second position, acquire an intensity of the reflected light at the third position from the light receiving sensor that receives the reflected light reflected at the third position, determine a second value which is a difference between the intensities of the reflected light at the second position and the third position, and determine a blood flow direction in the subject based on the first value and the second value. control the light output device to irradiate light to a first position of the subject and a second position of the subject located in a first direction with respect to the first position, acquire intensities of the reflected light at the first position and the second position from the light receiving sensor that receives the reflected light reflected at the first position and the second position, determine a first value which is a difference between the intensities of the reflected light at the first position and the second position, control the light output device to irradiate light to a third position of the subject located in a second direction opposite to the first direction with respect to the second position, acquire an intensity of the reflected light at the third position from the light receiving sensor that receives the reflected light reflected at the third position, determine a second value which is a difference between the intensities of the reflected light at the second position and the third position, and determine a blood flow direction in the subject based on the first value and the second value. <EOS>