Structure for preventing falling-down of cutter blade shaft of bush cutter

In a falling-down preventing structure for a cutter blade shaft (6) of a bush cutter, where the cutter blade shaft (6) is rotatably mounted in a vertical orientation in a cutter head case (1) via a pair of upper and lower bearings (4, 5) and where driving force is transmitted from a transmission shaft (15) to the cutter blade shaft (6) via driving and driven bevel gears (16, 7), a recessed portion (18) of a reduced diameter is provided on the cutter blade shaft (6) between the driven bevel gear (7) and the lower bearing (5), and a falling-down preventing pin (19) is inserted at its opposite end portions in the head case in such a manner as to extend across the recessed portion (18).