PURPOSE: To firmly assemble both frames for increasing the workability of assembling by only inserting the rear frame between both side frames from the above side, by providing pins at the lower part of the inside of both side frames constituting a desk and also a notch opening upward respectively.

CONSTITUTION: A groove 11 in which a rear frame 4 can be inserted from the upper side is longitudinally formed at the inside of a couple of right and left frames 1, 1 and pins 2 are provided back and forth at the lower part in the grooves 11 and further, notches 3 opening upward are provided at the upper side of the grooves 11. Moreover, notches 5 opening downward to fit the pins 2 at both lower sides are provided in the rear frame 4 and protrusive pins 6 to fit the notches 3 at the upper part are provided back and forth. Then the rear frame 4 is inserted between both side frames 1, 1 from the above, downward. In this way, the notches 5 of the rear side frame 4 are fitted into the pins 2 and also the pins 6 are fitted into the notches 3 opening upward. Then the firm assembling work of both frames 1, 4 is completed.