1. A work vehicle comprising:
a handle shaft coupled to a steering handle;
a steering shaft disposed on a same rotational axis as the handle shaft and provided for operating a steering mechanism;
a coupling mechanism that is in irregular-shape engagement with the handle shaft and the steering shaft for coupling and rotating the handle shaft and the steering shaft together about the rotational axis;
wherein the coupling mechanism includes:
a tubular boss member provided between the handle shaft and the steering shaft, with a shaft end portion of the handle shaft and a shaft end portion of the steering shaft being inserted through the tubular boss member,
a first key member attached to/between the boss member and the handle shaft, and
a second key member attached to/between the boss member and the steering shaft;
wherein a first engagement recess is formed at an outer circumferential portion of an irregular-shape engagement portion of the handle shaft relative to the coupling mechanism, whereas a first through hole extending through the boss member is formed at a portion of the boss member corresponding to the first engagement recess, an insertion hole corresponding to an outer circumferential shape of the first key member being constituted with establishment of communication between the first engagement recess and the first through hole;
wherein the first key member is inserted into the first engagement recess and the first through hole to engage with both the first engagement recess and the first through hole, thus restricting relative movement between the boss member and the handle shaft in the direction of rotational axis;
wherein a second engagement recess is formed at an outer circumferential portion of an irregular-shape engagement portion of the steering shaft relative to the coupling mechanism, whereas a second through hole extending through the boss member is formed at a portion

of the boss member corresponding to the second engagement recess, an insertion hole corresponding to an outer circumferential shape of the second key member being constituted with establishment of communication between the second engagement recess and the second through hole; and
wherein the second key member is inserted into the second engagement recess and the second through hole to engage with both the second engagement recess and the second through hole, thus restricting relative movement between the boss member and the steering shaft in the direction of rotational axis.
2. The work vehicle of claim 1, wherein the first engagement recess comprises a circumferential groove extending along the entire outer circumferential portion of the handle shaft and the second engagement recess comprises a circumferential groove extending along the entire outer circumferential portion of the steering shaft.
3. The work vehicle of claim 1 or 2, wherein:
the boss member is constituted of a C-shaped cross section having a cutout in a circumferential direction; and
the first key member and the second key member comprise a fastening tool which is inserted from the one side to the other side across the cutout therebetween to be fastened for fixing.
4. The work vehicle of any one of claims 1-3, wherein:
the first engagement recess and the first through hole are configured such that upon establishment of communication between the first engagement recess and the first through hole, there is formed an insertion hole extending like a tangential line at the outer circumferential portion of the irregular-shape engagement portion of the handle shaft; and
the second engagement recess and the second through hole are configured such that upon establishment of communication between the second engagement recess and the second through hole, there is formed an insertion hole extending like a tangential line at the outer circumferential portion of the irregular-shape engagement portion of the

steering shaft.