A reusability determination method for a gear during or after use, comprising: a retained γ phase measurement step (S1; S6) of measuring a content of retained γ phases in the gear; a retained γ phase change rate calculation step (S2; S7) of calculating a change rate of the content of the retained γ phases; a residual stress measurement step (S3; S6) of measuring residual stress applied to the gear; a residual stress ratio calculation step (S4; S7) of calculating a residual stress ratio to residual stress in an initial state of the gear; and a reusability determination step (S5; S9) of determining whether or not the gear is reusable on a basis of results of the calculation in the retained γ phase change rate calculation step (S2; S7) and the residual stress ratio calculation step (S4; S7), wherein, in the reusability determination step (S5; S9), it is determined that the gear is reusable where a state of the gear corresponds to a first phase (P 1) or a second phase (P 2), from among the first phase (P 1) in which a change rate of the residual stress increases as the change rate of the content of the retained γ phases decreases with reference to a state of the gear at time of start of use, the second phase (P 2) in which, after the first phase (P 1), the change rate of the residual stress increases steeply from that in the first phase (P 1) as the change rate of the content of the retained γ phase decreases, and a third phase (P 3) in which, after the second phase (P 2), the change rate of the residual stress decreases intermittently with respect to that in the second phase (P 2), and it is determined that the gear is not reusable where the state of the gear corresponds to the third phase (P 3). a retained γ phase measurement step (S1; S6) of measuring a content of retained γ phases in the gear; a retained γ phase change rate calculation step (S2; S7) of calculating a change rate of the content of the retained γ phases; a residual stress measurement step (S3; S6) of measuring residual stress applied to the gear; a residual stress ratio calculation step (S4; S7) of calculating a residual stress ratio to residual stress in an initial state of the gear; and a reusability determination step (S5; S9) of determining whether or not the gear is reusable on a basis of results of the calculation in the retained γ phase change rate calculation step (S2; S7) and the residual stress ratio calculation step (S4; S7), wherein, in the reusability determination step (S5; S9), it is determined that the gear is reusable where a state of the gear corresponds to a first phase (P 1) or a second phase (P 2), from among the first phase (P 1) in which a change rate of the residual stress increases as the change rate of the content of the retained γ phases decreases with reference to a state of the gear at time of start of use, the second phase (P 2) in which, after the first phase (P 1), the change rate of the residual stress increases steeply from that in the first phase (P 1) as the change rate of the content of the retained γ phase decreases, and a third phase (P 3) in which, after the second phase (P 2), the change rate of the residual stress decreases intermittently with respect to that in the second phase (P 2), and it is determined that the gear is not reusable where the state of the gear corresponds to the third phase (P 3).