The present invention provides a method for reducing web break caused in preparing, by continuous paper making, a fiber sheet containing a high content of functional inorganic substance. The composite fiber sheet manufacturing method includes: generating composite fibers composed of cellulosic fibers and inorganic particles by synthesizing the inorganic particles in slurry containing the cellulosic fibers; and continuously generating a sheet by supplying composite-fiber-containing slurry including the composite fibers to a continuous paper machine. In the composite fiber generating step, at least one of (i) slurry containing cellulosic fibers having a length of 1.2 mm to 2.0 mm in an amount of 16% or more in terms of length-weighted fiber length distribution (%) and (ii) slurry containing cellulosic fibers having a length of 1.2 mm to 3.2 mm in an amount of 30% or more in terms of length-weighted fiber length distribution (%) is used.