An engine startup device is provided having a helical-spline-engaging part where an output shaft (5) and a moving body (3) are coupled with each other, wherein a notch (37b, 38b) is formed on a power-transmitting-side tooth surface (37a, 38a) of at least either one of the output shaft and the moving body, and an angle formed by the notch with respect to a shaft direction is made smaller than the lead angle of a helical spline.