To prepare a reinforcing fiber sheet, hardly causing the seam slippage in the sheet, having a high toughness in the orienting direction of fibers and large interstices among the fibers and good in permeability to an adhesive.

This reinforcing fiber sheet is obtained by arranging covering auxiliary fibers 3 and 4 crossing respective riblike fiber groups in the longitudinal direction of the riblike fiber groups in at least one surface of the fiber sheet and binding the covering auxiliary fibers 3 and 4 between ridges 5 of the mutually adjacent fiber groups in a warp knitted textile design with binding auxiliary fibers 6 in the reinforcing fiber sheet comprising each fiber group, containing reinforcing fibers 1 or 2 arranged side by side as one unit and further arranged in the riblike shape with an interval kept therebetween.