PURPOSE: To freely vary swimming speed fitting in the pace of a principal by providing a microcomputer which controls flow speed corresponding to the change of the position of a swimmer measured by a measuring instrument.

CONSTITUTION: A water flow at prescribed flow speed is generated at a running water generator 2, and the swimmer 6 swims against the water flow. At this time, when the swimmer 6 is present at a position blocking a part between light emitting elements 4a, 4b, and 4c and light receiving elements 5a, 5b, and 5c, position detection signals are outputted from the light receivers 5a-5e in the measuring instrument 3, and those signals and the flow speed signal of water from the water flow generator 2 are inputted to the microcomputer 8. When the swimmer 6 progresses to the position blocking the part between the light emitting element 4d and the light receiving element 5b by increasing speed, the patterns of the position detection signals of the light receiving elements 5a-5e are varied, therefore, the change of the position of the swimmer 6 can be measured based on the above change, and also, the swimming speed is measured, thereby, the flow speed of the water can be controlled fitting in the swimming speed of the swimmer 6 by controlling the water flow genera tor 2 by the microcomputer 8.