An apparatus for drilling a work comprising: a housing (7 or 57), a shaft portion (3 or 53) extending from a holder (2 or 52) for holding a drilling tool (1 or 51) rotating around a first axis; a reduction gear means (14, 31, 32 or 64), including an input shaft (14b, 31a or 32a) connected to the shaft portion (3 or 53) and an output shaft (14a, 31d or 32f); a cylindrical member including an outer cylinder (19 or 69) and an inner cylinder (20 or 70) for rotatably containing the shaft portion (3 or 53) at an eccentric position, the cylindrical member (19, 20 or 69, 70) rotating around a second axis parallel to the first axis; a rotational drive means (15, 16 or 65) and a slide containing cylinder (17 or 67) for containing the shaft portion (3 or 53), the reduction gear means (14, 31, 32 or 64) and the cylindrical member (19, 20 or 69, 70) provided with a sliding structure for axially moving the shaft portion (3 or 53), the reduction gear means (14, 31, 32 or 64) and the cylindrical member (19, 20 or 69, 70), the slide containing cylinder (17 or 67) being slidable within the housing (7 or 57) characterized in that said rotational drive means (15, 16, or 65) is provided within the inner cylinder (20 or 70) of the cylindrical member (19, 20 or 69, 70), and in that said output shaft (14a, 31d or 32f) of the reduction gear means (14, 31, 32 or 64) is connected to the cylindrical member (19, 20 or 69, 70), such that the work is drilled by simultaneously performing a rotation around the first axis and a revolution around the second axis with rotating the drilling tool (1 or 51) by rotating the shaft portion (3 or 53) and simultaneously rotating the cylindrical member (19, 20 or 69, 70) connected to the output shaft (14a, 31d or 32f) of the reduction gear means (14, 31, 32 or 64) around the second axis at a predetermined reduction ratio by rotating the input shaft (14b, 31a or 32a) of the reduction gear means (14, 31, 32 or 64). a housing (7 or 57), a shaft portion (3 or 53) extending from a holder (2 or 52) for holding a drilling tool (1 or 51) rotating around a first axis; a reduction gear means (14, 31, 32 or 64), including an input shaft (14b, 31a or 32a) connected to the shaft portion (3 or 53) and an output shaft (14a, 31d or 32f); a cylindrical member including an outer cylinder (19 or 69) and an inner cylinder (20 or 70) for rotatably containing the shaft portion (3 or 53) at an eccentric position, the cylindrical member (19, 20 or 69, 70) rotating around a second axis parallel to the first axis; a rotational drive means (15, 16 or 65) and a slide containing cylinder (17 or 67) for containing the shaft portion (3 or 53), the reduction gear means (14, 31, 32 or 64) and the cylindrical member (19, 20 or 69, 70) provided with a sliding structure for axially moving the shaft portion (3 or 53), the reduction gear means (14, 31, 32 or 64) and the cylindrical member (19, 20 or 69, 70), the slide containing cylinder (17 or 67) being slidable within the housing (7 or 57) characterized in that said rotational drive means (15, 16, or 65) is provided within the inner cylinder (20 or 70) of the cylindrical member (19, 20 or 69, 70), and in that said output shaft (14a, 31d or 32f) of the reduction gear means (14, 31, 32 or 64) is connected to the cylindrical member (19, 20 or 69, 70), such that the work is drilled by simultaneously performing a rotation around the first axis and a revolution around the second axis with rotating the drilling tool (1 or 51) by rotating the shaft portion (3 or 53) and simultaneously rotating the cylindrical member (19, 20 or 69, 70) connected to the output shaft (14a, 31d or 32f) of the reduction gear means (14, 31, 32 or 64) around the second axis at a predetermined reduction ratio by rotating the input shaft (14b, 31a or 32a) of the reduction gear means (14, 31, 32 or 64).