A seat unit comprising: a seat and a backrest; a dining board having a body and an engaging portion located on a top face of the body; a reception member connected to the backrest and having a recessed area defined therein and a hook connected to the recessed area, two grooves respectively defined in two inner sides of the recessed area, and a guide device having multiple operation members connected to the dining board and slidably engaged with the grooves of the recessed area so as to slide the dining board up and down, the reception member located on a horizontal view level of a passenger behind the seat, the dining board being received in the recessed area of the reception member, characterised in that the operation members of the guide device comprise a first guide unit and a second guide unit, the first guide unit comprises two links which are inserted into two slots defined in the dining board, two respective lower ends of the two links are respectively and pivotably connected to two respective top ends of two legs, the two legs are connected to two recesses of the backrest, the second guide unit has the multiple operation members which are cooperated with the grooves of the reception member, the multiple operation members are connected to a left side and a right side of the dining board, so that when the engaging portion of the dining board is unhooked from the hook of the reception member, the dining board is pulled downward along the grooves and the links of the first guide unit are completely inserted into the slots, the dining board being then able to be pivoted downward to its horizontal position. a seat and a backrest; a dining board having a body and an engaging portion located on a top face of the body; a reception member connected to the backrest and having a recessed area defined therein and a hook connected to the recessed area, two grooves respectively defined in two inner sides of the recessed area, and a guide device having multiple operation members connected to the dining board and slidably engaged with the grooves of the recessed area so as to slide the dining board up and down, the reception member located on a horizontal view level of a passenger behind the seat, the dining board being received in the recessed area of the reception member, characterised in that the operation members of the guide device comprise a first guide unit and a second guide unit, the first guide unit comprises two links which are inserted into two slots defined in the dining board, two respective lower ends of the two links are respectively and pivotably connected to two respective top ends of two legs, the two legs are connected to two recesses of the backrest, the second guide unit has the multiple operation members which are cooperated with the grooves of the reception member, the multiple operation members are connected to a left side and a right side of the dining board, so that when the engaging portion of the dining board is unhooked from the hook of the reception member, the dining board is pulled downward along the grooves and the links of the first guide unit are completely inserted into the slots, the dining board being then able to be pivoted downward to its horizontal position.