An apparatus for producing a prepreg, for applying a coating liquid to a reinforcing fiber sheet, comprising: a coating section including: a liquid pool storing said coating liquid and having a portion whose cross-sectional area decreases continuously and vertically downward, and a narrowed section having a slit-like outlet in communication with the lower end of said liquid pool; a running mechanism for allowing said reinforcing fiber sheet to run vertically downward and be introduced into said coating section; and a take-up mechanism for taking up said reinforcing fiber sheet downward from said coating section; wherein said narrowed section is formed with wall constituent members (21a, 21b) opposed to each other in the thickness direction of said reinforcing fiber sheet, and wherein the apparatus further has an external force application mechanism (50a, 50b) for applying an external force to said wall constituent members (21a, 21b) in the thickness direction of said reinforcing fiber sheet, characterized in that the apparatus satisfies the following conditions i) or ii); i) the apparatus further comprises measurement devices (51a, 51b) that measure the thickness or mass of the prepreg downstream of the coating section and magnitude of said external force is controlled on the basis of the mass or thickness of the prepreg measured by said measurement devices (51a, 51b), ii) the apparatus further comprises measurement devices (52a, 52b) for measuring the amount of deformation of the wall constituent members (21a, 21b) and magnitude of said external force is controlled on the basis of the amount of deformation of the wall constituent members (21a, 21b) measured by said measurement devices (52a, 52b). a coating section including: a liquid pool storing said coating liquid and having a portion whose cross-sectional area decreases continuously and vertically downward, and a narrowed section having a slit-like outlet in communication with the lower end of said liquid pool; a liquid pool storing said coating liquid and having a portion whose cross-sectional area decreases continuously and vertically downward, and a narrowed section having a slit-like outlet in communication with the lower end of said liquid pool; a running mechanism for allowing said reinforcing fiber sheet to run vertically downward and be introduced into said coating section; and a take-up mechanism for taking up said reinforcing fiber sheet downward from said coating section; wherein said narrowed section is formed with wall constituent members (21a, 21b) opposed to each other in the thickness direction of said reinforcing fiber sheet, and wherein the apparatus further has an external force application mechanism (50a, 50b) for applying an external force to said wall constituent members (21a, 21b) in the thickness direction of said reinforcing fiber sheet, i) the apparatus further comprises measurement devices (51a, 51b) that measure the thickness or mass of the prepreg downstream of the coating section and magnitude of said external force is controlled on the basis of the mass or thickness of the prepreg measured by said measurement devices (51a, 51b), ii) the apparatus further comprises measurement devices (52a, 52b) for measuring the amount of deformation of the wall constituent members (21a, 21b) and magnitude of said external force is controlled on the basis of the amount of deformation of the wall constituent members (21a, 21b) measured by said measurement devices (52a, 52b).