A random mat comprising at least a chopped fiber bundle, wherein the chopped fiber bundle satisfies at least the following (a) to (d): (a) the chopped fiber bundle is a bundled aggregate of discontinuous reinforcing fibers obtained by cutting a partially separated fiber bundle prepared by alternately forming separation-processed sections, each of which is separated into a plurality of bundles, and not-separation-processed sections, along a lengthwise direction of a fiber bundle comprising a plurality of reinforcing fibers; (b) the chopped fiber bundle has a first transition section in which the number of the reinforcing fibers in a cross section of the fiber bundle in a direction perpendicular to the lengthwise direction of the fiber bundle is increased from a first tip which is one tip in the lengthwise direction of the fiber bundle, toward a second tip which is the other tip, and a second transition section in which the number of the reinforcing fibers in the cross section of the fiber bundle is increased from the second tip toward the first tip; (c) between the first transition section and the second transition section, along the lengthwise direction of the fiber bundle, an unchanging section in which the number of the reinforcing fibers in the cross section of the fiber bundle is invariable is provided, and one end surface of the unchanging section coincides with a first terminal end surface which is a terminal end at a side opposite to the first tip of the first transition section and the other end surface of the unchanging section coincides with a second terminal end surface which is a terminal end at a side opposite to the second tip of the second transition section, or the first terminal end surface and the second terminal end surface coincide directly with each other; and (d) a change amount of the total cross-sectional area of the reinforcing fibers in the cross section of the fiber bundle between the first tip and the second tip is 0.05mm 2 or less per a length of 1 mm in the lengthwise direction of the fiber bundle, wherein the partially separated fiber bundle is cut at an angle θ (4° ≤ θ ≤ 25°) with respect to the lengthwise direction of the fiber bundle when obtaining the chopped fiber bundle. (a) the chopped fiber bundle is a bundled aggregate of discontinuous reinforcing fibers obtained by cutting a partially separated fiber bundle prepared by alternately forming separation-processed sections, each of which is separated into a plurality of bundles, and not-separation-processed sections, along a lengthwise direction of a fiber bundle comprising a plurality of reinforcing fibers; (b) the chopped fiber bundle has a first transition section in which the number of the reinforcing fibers in a cross section of the fiber bundle in a direction perpendicular to the lengthwise direction of the fiber bundle is increased from a first tip which is one tip in the lengthwise direction of the fiber bundle, toward a second tip which is the other tip, and a second transition section in which the number of the reinforcing fibers in the cross section of the fiber bundle is increased from the second tip toward the first tip; (c) between the first transition section and the second transition section, along the lengthwise direction of the fiber bundle, an unchanging section in which the number of the reinforcing fibers in the cross section of the fiber bundle is invariable is provided, and one end surface of the unchanging section coincides with a first terminal end surface which is a terminal end at a side opposite to the first tip of the first transition section and the other end surface of the unchanging section coincides with a second terminal end surface which is a terminal end at a side opposite to the second tip of the second transition section, or the first terminal end surface and the second terminal end surface coincide directly with each other; and (d) a change amount of the total cross-sectional area of the reinforcing fibers in the cross section of the fiber bundle between the first tip and the second tip is 0.05mm 2 or less per a length of 1 mm in the lengthwise direction of the fiber bundle,