1. A character string placement device comprising: an evaluation function calculator that calculates a plurality of evaluation function values, each of which shows an evaluation of placement of a character string placed at each of a plurality of candidate points on a node series; and a minimum evaluation function value storer that determines at which of said plurality of candidate points to place the character string on a basis of comparison between said plurality of evaluation function values calculated by said evaluation function calculator; wherein said evaluation function calculator includes: a character gap function calculator that calculates a character gap function value for evaluating a distance between the character string placed at each of said plurality of candidate points and another character string placed at another node series, which is different from the node series on which said plurality of candidate points are placed; an angle function calculator that calculates an angle function value showing an evaluation of an angle of the node series along which the character string is placed at each of said plurality of candidate points; and an evaluation function value calculator that calculates said evaluation function value of the character string placed at each of said plurality of candidate points based on F=log P(R string _ idx |S string _ idx )+log P(S string _ idx |{S string _ idx-1 }), wherein F is said evaluation function value, P(R string _ idx |S string _ idx ) is said angle function value, and P(S string _ idx |{S string _ idx-1 }) is said character gap function value.