1. A gesture detecting method implemented by a computer programmed as a gesture-detecting apparatus, the method comprising: calculating, by the computer, a velocity vector of a specific part of an object gesturing, from time series data of coordinates of the specific part, 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 a representative velocity vector from velocity vectors in each dynamic period; storing each representative velocity vector corresponding to each dynamic period in a storage unit; calculating a first intersection angle of two representative velocity vectors stored in the storage unit, the two representative velocity vectors corresponding to two successive dynamic periods; and detecting that a motion of the specific part is a reciprocating motion when the first intersection angle is larger than a first threshold value.