1. A gesture detecting method comprising: preparing a time series data of coordinates of a specific part of an object gesturing; calculating a velocity vector of the specific part from the time series data of coordinates, to obtain a time-series velocity vectors; detecting dynamic periods when displacement per unit time of the specific part is larger than a displacement threshold value, based on the time-series velocity vectors; acquiring each representative velocity vector from velocity vectors in each dynamic period; storing the each representative velocity vector corresponding to the each dynamic period in a memory; calculating a first intersection angle of two representative velocity vectors stored in the memory, the two representative velocity vectors corresponding to two successive dynamic periods; and detecting that the motion of the specific part is a reciprocating motion when the first intersection angle is larger than a first threshold value.