PURPOSE: To allow always the uniform cooling of a strip by detecting the delay in the response of a solenoid valve of a laminar flow header by using an injec tion detecting sensor and supplying cooling water to the front end of the strip at the timing of high accuracy.

CONSTITUTION: The solenoid valve 7 is opened by a command 4a of a computer 4 to send the cooling water from a cooling water source 6 to the laminar flow header 1 and to supply the cooling water from the injection ports 2a of injection pipes 2 to the front end of the strip 11. The delay t in the response from the command 4a to the injection is adjusted by opening the solenoid valve 7 in precedence hy as much as t, by which the cooling water is exactly sup plied to the front end of the strip 11. The delay t in response is checked at all times hy the injection sensor 3 on increasing of this delay while the strip 11 is kept cooled continuously. A change in t is automatically corrected by preceding the timing to open the solenoid Valve 7 by as much as the difference from the initial reference value according to this change. The cooling water is so controlled as to fall onto the front end of the strip 11 at all times with the high accuracy in this way. The coil having stable quality is thus obtd.