Small-diameter deep hole drill and a micro deep hole drilling method

A small-diameter drill for deep hole drilling is provided which is capable of inhibiting the cutting chips from getting bitten and producing a straightness for a drilled hole, thus suitable for drilling a hole having a depth (L) which is 15 or more times the diameter (D) of a drilled hole. The small-diameter drill has a diameter which is 1 mm or less and a drill section including cutting edge(s) and flute(s) . The length of the flute is at least 5 times and at most 10 times the drill diameter . The diameter of the drill section , after being reduced in diameter from the cutting edge toward the rear of the drill section , is expanded in diameter so that the outer diameter at the end of the flute becomes at least 0.9 times and at most 0.98 times the drill diameter . An expanded-diameter portion is provided between the drill section and a neck section connected to a shank and is expended in diameter from the outer diameter at the end of the flute up to 1 or less times the drill diameter . The diameter of the neck section is smaller than the drill diameter , and the length of the neck section is 10 or more times the drill diameter