PURPOSE: To uniformize the coated thickness of a liq. resin on a base and to produce a stabilized-quality impregnated sheet by controlling the opening degree of a control valve in accordance with a signal from a measuring part to fix the liq. level.

CONSTITUTION: The liq. resin tank 1 contg. a liq. resin 7 to be supplied between a backup roll 5 and a doctor knife 6, an ultrasonic wave transmitter-receiver 2 for emitting an ultrasonic wave toward the liq. resin surface in the tank 1 and receiving the reflected wave, the measuring part 3 for measuring the height of the liq. surface connected to the transmitter-receiver and the valve 4 capable of controlling the supply of the resin 7 to the tank 1 and connected to the output terminal of the part 3 are provided. The opening degree of the valve 4 is controlled in accordance with the signal from the part 3 to fix the liq. surface. Accordingly, the liq. surface in the tank 1 is kept at a fixed position, the supply of the resin 7 to the doctor knife 6 is stabilized, and the coated thickness on the sheet base 10 is uniformized.