PURPOSE: To improve marking precision, and marking workability by composing a guide slider by jointing a pair of support poles having a level using a crosspiece, and providing a marker to slide on the guide slider.

CONSTITUTION: A guide slider 1 is composed by jointing a pair of support poles 3, 3 by a crosspiece 2 through joint mechanisms 4, 4 capable of adjusting an angle of the crosspiece 2. The support pole 3 is formed of a rectangular pole material, a vertical slide groove to guide the joint mechanism 4 on the crosspiece 2 is formed in it, and an aperture side surface of the vertical slide groove is formed in a slide surface 10 for a marker 30. Levels 11, 11 are installed on upper ends of the respective support poles 3. The crosspiece 2 is formed rectangular, a lateral slide groove is formed in it, and a slide surface 6 for the marker 30 is provided. The marker 30 is composed by providing a holding body 35 having a marking member 34 at a forward end between a pair of guide plates 31. A slide surface 36 of it is applied to slide on the slide surfaces 10, 6 in the support pole 3 and the crosspiece 2 to move for marking.