To automate the feeding of a hydraulic core drill having a bit that is supported to be able to ascend and descend along a support, a hydraulic motor that rotates/drives the bit and a feed motor that rotates/drives a pinion that is in mesh with a rack installed to the support and thus to attain laborsaving.

The automatic feeder comprises a pressure sensor 8 that detects the pressure of the compressed oil sent to the hydraulic motor and a controlling device 9 that controls the rotational frequency or feeding force of the feed motor so that the pressure detected by the pressure sensor 8 may be maintained at a set value. The controlling device 9 makes the feed motor rotate inversely and makes the bit 1 climb when the bit 1 locks with a concrete structure 17 and the pressure reaches an upper limit set value, and, when the pressure of the compressed oil lowered by this reaches a first lower limit set value, makes the feed motor rotate positively with a lowered rotational frequency and after a set time makes it resume the original rotational frequency.