A fiber structure comprising an extra-fine fiber layer spreading in a plane direction, and a substrate layer adjoining the extra-fine fiber layer, wherein the extra-fine fiber layer comprises extra-fine fibers having a number average single fiber diameter as measured as set out in the specification of 5 μm or less; the substrate layer comprises non-extra-fine fibers having a number average single fiber diameter as measured as set out in the specification of 7 μm or more; in a cross section along a thickness direction of the fiber structure, the substrate layer contains mixture portions in each of which some of the extra-fine fibers pushed between the non-extra-fine fibers are widened in a crosswise direction; and where the substrate layer is equally divided into three segments and each of the segments is denoted sequentially from the extra-fine fiber layer side as a proximal region, a central region, and a distal region, at least a part of the pushed extra-fine fibers from the extra-fine fiber layer reach the distal region, wherein the fiber structure is an entangled product of one or more extra-fine fiber layers formed of the extra-fine fibers with one or more non-extra-fine fiber layers formed of the non-extra-fine fibers, the extra-fine fiber layer is a nonwoven fabric, the non-extra-fine fiber layer is a nonwoven fabric, the fibers constituting the substrate layer contain at least one species selected from the group consisting of polyolefin-based fibers, polyester-based fibers, acrylic-based fibers, heat-resistant fibers not completely molten after being heated at 200°C for 3 hours, and composite fibers thereof in a proportion of 90% by mass or more, wherein the heat-resistant fibers are at least one of polyetheretherketone fibers, polyetherketone fibers, polyetherketoneketone fibers, polyphenylene sulfide fibers, aromatic polyamide fibers, aramid fibers, polyimide-based fibers, polyetherimide fibers, polyamideimide fibers, polybenzoxazole fibers, polybenzoimidazole fibers, polybenzothiazole fibers, polytetrafluoroethylene fibers, melamine fibers, and novoloid fibers, and the fiber structure has a basis weight of 20 to 180 g/m 2. the extra-fine fiber layer comprises extra-fine fibers having a number average single fiber diameter as measured as set out in the specification of 5 μm or less; the substrate layer comprises non-extra-fine fibers having a number average single fiber diameter as measured as set out in the specification of 7 μm or more; in a cross section along a thickness direction of the fiber structure, the substrate layer contains mixture portions in each of which some of the extra-fine fibers pushed between the non-extra-fine fibers are widened in a crosswise direction; and where the substrate layer is equally divided into three segments and each of the segments is denoted sequentially from the extra-fine fiber layer side as a proximal region, a central region, and a distal region, at least a part of the pushed extra-fine fibers from the extra-fine fiber layer reach the distal region, wherein the fiber structure is an entangled product of one or more extra-fine fiber layers formed of the extra-fine fibers with one or more non-extra-fine fiber layers formed of the non-extra-fine fibers, the extra-fine fiber layer is a nonwoven fabric, the non-extra-fine fiber layer is a nonwoven fabric, the fibers constituting the substrate layer contain at least one species selected from the group consisting of polyolefin-based fibers, polyester-based fibers, acrylic-based fibers, heat-resistant fibers not completely molten after being heated at 200°C for 3 hours, and composite fibers thereof in a proportion of 90% by mass or more, wherein the heat-resistant fibers are at least one of polyetheretherketone fibers, polyetherketone fibers, polyetherketoneketone fibers, polyphenylene sulfide fibers, aromatic polyamide fibers, aramid fibers, polyimide-based fibers, polyetherimide fibers, polyamideimide fibers, polybenzoxazole fibers, polybenzoimidazole fibers, polybenzothiazole fibers, polytetrafluoroethylene fibers, melamine fibers, and novoloid fibers, and the fiber structure has a basis weight of 20 to 180 g/m 2.