The half-clutch state decision device of the present invention extracts a high-frequency component and a low-frequency component of an engine rotation speed through filtering when the engine rotation speed has dropped while a vehicle is stopped, and calculates a locus length of the high-frequency component and a locus length of the low-frequency component through accumulation thereof. Furthermore, it is decided whether a half-clutch state exists from a relationship between the calculated locus lengths of the high-frequency and low-frequency components. Specifically, it uses a two-dimensional map (FIG. 5) having the locus length of the high-frequency component and the locus length of the low-frequency component as parameters and wherein a half-clutch decision region is set, plots the locus length of each component on that two-dimensional map, and decides whether a half-clutch state exists based on whether that plot point is inside the half-clutch decision region.