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 ). <EOS>