PURPOSE: To prevent the decrease of the thickness of a fine tube owing to erosion and to reduce the drum size of a heat-exchanger through reduction of the number of fine tubes, in a heat-exchanger which cools the interior of the fine tube or through which fluid for heating is caused to flow.

CONSTITUTION: A flow rate control device for a heat-exchanger comprises a bypass pipe 4 bypassing a heat-exchanger 1 having a plurality of fine tubes 1a, a flow rate control valve 2 arranged in the bypass pipe 4, a flow rate detector 5 located in a main piping 3 or 3' in a position situated upper stream or downstream from the heat-exchanger 1, and a flow rate controller 6 to open the flow rate control valve 2 when a flow rate of fluid A detected by the flow rate detector 5 attains a given value. When a flow rate of the fluid A passing through the heat-exchanger 1 exceeds a given value, the surplus fluid A is caused to flow through the bypass pipe 4, and velocity of flow of the fluid A flowing through the fine tube 1a is suppressed to a value below a given upper limit value.