DESCRIPTION PARTICLE BOARD Technical Field [0001] The present invention relates to a particle board. Background Art [0002] There has been known that an adhesive such as a synthetic resin is coated onto particles (small pieces) obtained by pulverizing a plant-based material, including wood, to thereby form a mixture of the particles and the adhesive, and the mixture is thermocompression-molded to manufacture a particle board. Such a particle board has attracted attention as an alternative material of plywood, and thus it is used as an interior building material such as a door panel, a sliding door, and a partition, and recently used as a floor material (e.g., see Patent Literature 1). However, since the particle board is largely expanded or contracted in dimension by absorption and desorption of moisture in comparison with the plywood, construction may be limited in adhesion, for example, or a material blocking humidity may be required. Thus, it has been expected to improve dimensional stability against absorption and desorption of moisture. [0003] Meanwhile, there has been used, as the interior building material, a fiber board manufactured by thermocompression-molding a resin composite continuous fiber mat in which adhesive is uniformly scattered to continuous fibers (e.g., see Patent Literature 2). The fiber board has generally a better dimensional stability against absorption and desorption of moisture in comparison with the particle board. Citation List Patent Literatures [0004] Patent Literature 1: JP 2010-150912 A Patent Literature 2: JP 2009-172929 A Summary of Invention Technical Problem [0005] However, in a fiber board, its surface smoothness is generally inferior to that of a particle board. Thus, even if the particle board can be applied as an alternative material of plywood in the interior building material and so on, the fiber board may not be applicable. [0006] In view of the above circumstances, an object of the present invention is to provide a particle board having surface smoothness and small dimensional variations due to absorption and desorption of moisture. Solution to Problem [0007] In order to solve the above problem, a particle board of the present invention is formed by thermocompression-molding a mixture of plant-based particles and an adhesive. In this particle board, plant-based fibers are contained in at least a surface layer, and an average fiber length of the plant-based fibers in the surface layer is not less than 0.3 times and not more than 3.0 times an average length of the particles in the surface layer. [0008] In the particle board, a content of the plant-based fibers in the surface layer is preferably not less than 10% by weight and not more than 30% by weight based on the particles in the surface layer. [0009] In the particle board, it is preferable that the plant-based fibers are cut by being held between a pair of cutting blades. [0010] In the particle board, it is preferable that the adhesive is an isocyanate resin adhesive or a phenol resin adhesive. [0011] In the particle board, it is preferable that as the adhesive, a melamine resin adhesive is further used together. [0012] In the particle board, it is preferable that the plant-based fibers are non-wood fibers. Advantageous Effects of Invention [0013] In the particle board of the present invention, plant-based fibers having a predetermined average fiber length are contained in a surface layer. Consequently, a particle board having surface smoothness and small dimensional variations due to absorption and desorption of moisture can be obtained. Brief Description of Drawings [0014] Fig. 1 is a cross-sectional view showing an example of a particle board. Fig. 2 is an explanatory view showing cutting of a fiber bundle using a cutting machine. Figs. 3(a) and 3(b) are pattern diagrams of a relevant portion of a mesh used in manufacturing of the particle board of the present invention. Fig. 3(a) is a mesh having circular holes, and Fig. 3(b) is a mesh having square holes. Description of Embodiments [0015] Hereinafter, an embodiment of the present invention will be described in detail. [0016] A particle board of the present embodiment is formed into a plate shape to have a constant area and thicl o 3 CD 2 Magnification of average fiber length of plant-based fibers with respect to average length of particles of surface layer (average fiber length of plant-based fibers) 0.3 times (1 mm) 1.4 times (5 mm) 3 times (10 mm) 0.2 times (0.7 mm) 3.4 times (12 mm) No plant-based fibers Content of plant-based fibers with respect to particles of surface layer 25% by weight 25% by weight 25% by weight 25% by weight 25% by weight No plant-based fibers Compounding ratio of adhesive to particle of surface layer (kind of adhesive) 5% by weight (isocyanate resin adhesive) 5% by weight (melamine resin adhesive) 5% by weight (isocyanate resin adhesive) 5% by weight (melamine resin adhesive) 5% by weight (isocyanate resin adhesive) 5% by weight (melamine resin adhesive) 5% by weight (isocyanate resin adhesive) 5% by weight (melamine resin adhesive) 5% by weight (isocyanate resin adhesive) 5% by weight (melamine resin adhesive) 5% by weight (isocyanate resin adhesive) 5% by weight (melamine resin adhesive) Method of cutting plant-based fibers Pair of cutting blades Pair of cutting blades Pair of cutting blades Pair of cutting blades Pair of cutting blades ^^^^-^^ •*—* 3 (0 a? o '•4—> TO CO .5 Uniform mixability of mixture for surface layer formation O O O O X Dimensional change ratio of particle board 0.28% 0.23% 0.27% 0.37% Measurement is not performed because particle board cannot be manufactured 0.40% Dimensional variability due to absorption and desorption of moisture O © O X Evaluation is not performed because particle board cannot be manufactured 15 [Table 2] No. Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 c .g '■u c o o D) c ■c 3 D C