PURPOSE: To improve the reliability for a step worked hole by setting the optimum cut-in quantity in the initial time to 4-6 times of the twist drill diameter, 2-3 times in the second cut-in quantity, and 1.5-3 times in the third cut-in quantity and the succeeding, and making the core thickness small on the top edge side and large on the root side, and determining the shape of the twist drill top edge.

CONSTITUTION: As for a twist drill 3 for printed circuit board which carries out drilling, removing chips by blowing air at a high speed to the twist drill, the core thickness is made small on the top edge side and made large on the root side. The core thickness at the top edge is 10-25% of the twist drill diameter, groove width ratio is 1.2-2.2, top edge angle is 118-135°, cutter second angle is 15-20°, third angle is 25-30°, and a twist angle is 20-35°, and the groove sectional area at the taper end is 20% or less of the body sectional area, and a part free from groove is formed between the groove terminal edge and the body terminal edge, and the body length is set at a substrate plate thickness +. The twist drill diameter is 0.3-0.5mm, body length is 6.4-7.1mm, top edge side twist angle is 24-35°, and 20-24° at the groove terminal edge part of the root. The initial cut-in quantity is made about five times of the twist drill diameter, about 2.5 times in the second cut-in quantity, and about two times in the third cut-in quantity. Accordingly, the twist drill free from breakage and having the superior hole position and quality can be obtained.