There is provided an individual-organism counting device being capable of determining the number of even fine individual organism, such as shrimp larvae, with high accuracy and also having a simple structure and a reduced size, while being less prone to debilitate and extinguish the individual organism. In order to realize this, inclined flow channels 10A and 10B inclined in opposite directions are placed in two stages which are upper and lower stages, and sea water in a tank 21 is circulated through the inclined flow channels 10A and 10B. An aggregation of individual organism 60 to be counted is introduced into an upstream portion of the upper-stage inclined flow channel 10A. The introduced individual organism 60 are dispersed while they proceed from the upper-stage inclined flow channel 10A to the lower-stage inclined flow channel 10B, and, further, they are counted by photographing with a camera 40 and by analyses of images captured by an image analysis portion 50, in a measurement portion 18B provided halfway through the inclined flow channel 10B. There are provided stem-shaped liquid reservoirs 14A and 14B having bottom surfaces which are inclined upwardly toward their downstream sides with inclination angles gradually increasing with increasing distance from the opposite sides to the center, downstream from an individual-organism introducing portion 12A in the inclined flow channel 10A and upstream from the measurement portion 18B in the inclined flow channel 10B to promote the dispersion of the individual organism 60.
