PURPOSE: To enable to count the number of fry, simply, quickly, and in high accuracy, by giving light downward the measuring stream and counting the shadow of fry caused with the photo sensors provided in the stream.

CONSTITUTION: Taking the angle of the arm 7 and the light source 5 intersecting the measuring stream 4 as θ, the velocity of water of the stream 4 as v1, and running speed of the photo sensor as v2, then θ=tan-1(v1/v2). If θ is set like this, the sensor 8 is as if it were at standstill to water flow. When the sensor 8 finishes the atream 4 with crossing over, the reset switch 10 immediately operates and it returns to original position in high speed (near the sprocket B1). Next, the sensor 8 detects the shadow of D for the next fry row delayed by the length of body of fry D and counts it. Thus, every time when the sensor 8 detects the number of projections of D, it is integrated with the counter 11 to accumulate the number of every fry row.