Provided are a simple fish counting device, fish counting method, fish count prediction device, fish count prediction method, fish counting system, and fish count prediction system. The fish counting device is characterized in that it includes: a measurement image acquisition means (11) that acquires multiple measurement images over time for a measurement subject region in a passage region through which a fluid containing fish passes； a fish position acquisition means (21) that acquires the positions of the fish for each of the multiple measurement images； a fish line-of-movement calculation means (22) that, on the basis of the positions of the fish in the multiple measurement images, calculates the line of movement for the fish, that is, the trajectory along which the fish are moving； and a fish counting means (23) that, on the basis of the lines of movement of the fish in the multiple measurement images, counts the number of fish.提供簡易的魚計數裝置、魚計數方法、魚數之預測裝置、魚數之預測方法、魚計數系統及魚數預測系統。本發明的魚計數裝置，其特徵在於包括測量影像取得手段11，關於包含魚的流體通過的通過區域中的測量對象區域，依時取得複數的測量影像；魚位置取得手段21，關於各個上述複數的測量影像，取得上述魚的位置；魚移動線算出手段22，根據上述複數的測量影像中的上述魚的位置，算出上述魚移動的軌跡之魚的移動線；以及魚計數手段23，根據上述複數的測量影像中的上述魚的移動線，計算上述魚數。
