PURPOSE: To control the quantities of abrasive grain fluid passing through respective holes of extremely small diameter bored in a work to be uniform so as to uniformalize modified processing and finishing accuracy of these holes.

CONSTITUTION: A processor is so formed that a hollow work W in which a plurality of holes Wa (fuel injection holes) of extremely small diameter are bored at its top end is set in a work holder 12, abrasive grain fluid F is supplied to the inside of the work W by a prescribed supply device 14, and the shapes of respective holes Wa are processed for modification by passing abrasive grain fluid F so as to be finished. And this device is provided with a conversion block 20 which has channels 22 for communicating to respective holes Wa of the work W individually and in which intervals between respective adjacent opening positions of the channels 22 are set sufficiently larger than those of respective holes Wa and flow regulating mechanism 30 which can uniformly control the quantities of the abrasive grain fluid F passing through respective holes Wa positioned respectively in the opening points of respective channels 22 in the conversion block 20.