To reduce the size of a traveling reduction gear as the arrangement of a planetary gear 28 and a tip end of a fixed side housing 21 is a cause of a size increase of the traveling reduction gear in the axial direction.

In the traveling reduction gear 1, a planetary gear type speed reduction mechanism 5 includes a second sun gear 57, a plurality of second planetary gears 58 disposed around the second sun gear 57 for transmitting rotation of the second sun gear 57 to a rotary side housing 4, and a second carrier 59 that supports a shaft part 58a of the plurality of the second planetary gears 58 and that is fixed to the tip end 31 of the fixed side housing 3. The second carrier 59 has a hole formed therein having a radius R4 smaller than a distance from the center of the second sun gear 57 to the inside of the shaft part 58a of the second planetary gears 58. A second tip end 34 of the tip end 31 of the fixed side housing 3 is inserted into the hole of the second carrier 59 and is positioned on the radially inner side of the plurality of second planetary gears 58.