A height adjustment device is provided that has a first chassis having an adjustment hole, an adjustment screw that stands when threadedly engaged with the adjustment hole in the first chassis, a second chassis locked by the adjustment screw so as to have a preset separation distance from the first chassis, and biasing means for biasing the first chassis and the second chassis in the direction of attaining separation therebetween. The adjustment screw has a tip portion for forming a female thread in the adjustment hole when rotated and inserted into the adjustment hole, a threaded portion having a male thread threadedly engaged with the female thread formed by the tip portion, and a head portion for locking the second chassis against the direction of biasing by the biasing means, in a nearly linear state. The distance between the end of the head portion of the adjustment screw on the male thread side and the end of the tip portion on the male thread side is made larger than the sum of the length of the inside diameter portion of the adjustment hole in which the female thread is formed, the maximum value of the distance between the first chassis and the second chassis and the thickness of the second chassis.