Abstract: A method of manufacturing a hollow circuit substrate, of two metal sheets brazed to each other in a laminated state with a bulging hollow circuit for ed between the two metal sheets. Of the upper and lower metal sheets for forming the hollow circuit, a circuit-forming bulging portion is formed in the upper metal sheet. A flux suspension is applied by screen printing to the upper surface of the lower metal sheet so as not to overlap the circuit-forming bulging portion to form flux films. The two metal sheets are stacked on each other so as to close off the opening of the circuit-forming bulging portion and are brazed to each other. This method prevents flux from remaining in a hollow circuit of a manufactured hollow circuit substrate.