To provide a highly versatile horizontal displacement device or base isolation device which can cope with various sizes or weights of targets loaded onto the device, is simplified in structure and has a small number of kinds and a reduced number of component parts, and a method of assembling the device.

A first horizontal displacement unit 30 having an overall width W is composed of upper and lower plates 32, 33 which are relatively movable in the longitudinal direction, each plate having a longitudinal length L and a width D. A second horizontal displacement unit 40 having an overall width L is composed of upper and lower plates 42, 43 each having a longitudinal length W and a width D'. The first horizontal displacement unit 30 and the second horizontal displacement unit 40 are integrally assembled so that the longitudinal directions thereof are perpendicular to each other. The width D is equal to the width D'. Balls 6 are employed as rolling elements. Ball grooves 34, 44 are made to intersect each other at an angle θ so that the balls slide and roll in the grooves. The ball grooves are respectively the deepest at their central portions 2c, 3c. Horizontal displacement component parts 31, 41 having a plurality of longitudinal lengths L are prepared and four or more component parts are selected and assembled so as to correspond to a required size. The standard longitudinal length is set at 450 mm and other longitudinal lengths are set to be larger than the standard one by 150 mm or multiples of 150 mm.