PURPOSE:To simultaneously form an oblique flocking hole by an injection mold by obliquely inserting a core for the flocking hole wherein at least one part is freely curved into the cavity of a forming mold so that the core is made a nearly reverse V-type, introducing and solidifying material and thereafter obliquely pulling out the core and opening the forming mold. CONSTITUTION:When resin material introduced into a cavity 4 is cooled and solidified, mold opening action is started. A core 15 is guided into a core guide hole 23 together with the backing plates 11, 12 and an ejector pin 18 by guide of a guide pin 10 and guide bushes 8, 9. Especially core parts 21a, 21b are guided by oblique guide holes 23a, 23b and obliquely inserted and retreated. At this time, a movable mold 2 is in a state of being pressed to a fixed mold 3 by a core ejecting spring 24. On the other hand, while flexible parts 22a, 22b of the core 15 are curved respectively, the core is smoothly pulled out from the oblique flocking hole of the molded brush base product without causing unreasonableness in the core parts 21a, 21b.