Trim the celery and cut it into 5 cm (2 inch) lengths. Peel the celeriac and cut the flesh into cubes. Separate the lettuce into leaves. Juice the celery, celeriac, lettuce and spinach, alternating the ingredients so that the lettuce and spinach leaves do not clog the machine.
Transfer the juice to a food processor or blender, add a couple of ice cubes and process briefly.
Pour the juice into a tall glass and serve immediately.