PURPOSE: To enable the installation of each spindle inside a retaining tube therefor to the almost full extent in the direction of length thereof and thus to enable the firm support of each spindle by a spindle holder free from sideward vibrations.

CONSTITUTION: The rotation of the main shaft is transmitted from a main-shaft gear 3 to each intermediate gear 19 by the 1st driven gear 20 and further transmitted to each of spindles 32 through the intermediary of the 2nd driven gear 21 and a spindle gear 41 engaged with the former. Each spindle 32 is installed inside the retaining tube 31 of a spindle holder 27 almost entirely in the direction of length thereof. Therefore, the sideward force acting on the tip of each drill 43 in boring is supported securely by the spindle 32.