An assist mechanism of a steering apparatus for a working machine includes right and left operational arm members for selectively engaging and disengaging right and left clutches. Right and left cam grooves are formed in the right and left operational arm members, respectively. A bar-shaped member has opposite ends disposed within the right cam groove and the left cam groove, respectively. A compression spring is disposed along a central axis of the bar-shaped member for urging the right and left operational arm members in directions away from each other.