1. A processing apparatus for creating an avatar, comprising: a skeleton database to store statistics of skeleton sizes with respect to reference poses of a plurality of users; a function determiner which determines a difference function representing a difference between a forward kinematics function, the forward kinematics function regarding joints of the avatar with respect to a reference pose of a target user, and positions of sensors attached to the target user, and to determine a skeleton prior function based on the statistics of the skeleton sizes with respect to the reference poses of the plurality of users, wherein the function determiner uses at least one processor; a calculator which calculates skeleton sizes of the joints of the avatar, and local coordinates corresponding to the sensors using at least one processor; and a display which displays the avatar based on the calculated skeleton sizes of the joints and the calculated local coordinates, wherein the skeleton database is updated periodically or non-periodically according to changes of the reference poses, wherein the calculator performs the calculation of skeleton sizes of the joints and local coordinates using the equation arg ⁢ ⁢ min s , z , λ n ⁢  f ⁡ ( s , z ; q ~ t ) - c ~ t  2 +  s - e 0 - ∑ n = 1 B ⁢ ⁢ λ n ⁢ e n  , wherein, the difference function (∥ƒ(s, z; {tilde over (q)} t )−{tilde over (c)} t ∥ 2 ) represents the difference between the forward kinematics function (ƒ(s, z; {tilde over (q)} t )), the forward kinematics function regarding the joints of the avatar with respect to the reference pose of the target user, and the positions ({tilde over (c)} t ) of the sensors attached to the target user, wherein s denotes the skeleton size of the reference pose of the target user, z denotes the local coordinate of the sensor, and {tilde over (q)} t denotes the joint angle with respect to the reference pose of the target user, and wherein e 0 denotes a mean, e n denotes an eigen vector, λ n denotes an eigen value calculated from the statistics of the skeleton sizes with respect to the reference poses of the plurality of users, and B denotes a number of main eigen vectors among the eigen vectors e n arranged in order of importance.