PURPOSE: To provide a piercing rolling mill which can execute piercing rolling of a tube stock having large thickness/outside diameter ratio without incurring generation of tail shrink, erosion of plug tip part and deformation of a mandrel bar to support plug.

CONSTITUTION: A piercing mill is provided with a plug 3 which has a reducing part 31 to reduce outside diameter roughly in linear in continuing to a head part 30, having hemisphere tip part and an equal diameter part 32 having roughly constant outside diameter at this sequence in the moving direction of tube stock 4, and inclined rolls 1, 1 having a second expanding part 12 connecting to a gorge part 10 with increasing its diameter in the moving direction of tube stock 4 in the range coinciding with the reducing part 31 of plug 3. The face angle α2 of second expanding part 12 and the face angle θ of reducing part 31 is set to roughly equal, a tube stock 4 held between the inclined rolls 1, 1 and plug 3 is reduced while rolling down between the second expanding part 12 and reducing part 31.