the variable pressure chamber 6 as shown in fig2 in the embodiment insertion holes 36 are formed at four corners of a flat part of the rear shell 3 the insertion holes 36 are disposed such that when the booster is attached to a support member 30 of a vehicle a total of four linking bolts including two upper linking bolts 33 located on an upper side and two lower linking bolts 34 located on a lower side protrude protruding parts 37 one right side on the drawing of the upper linking bolts 33 is the tie rod bolt 13 described above and the other left side on the drawing is a non tie rod bolt the tie rod bolt 13 is disposed as the lower linking bolt 34 located on a diagonal line of an extending bolt 35 of the upper linking bolt 33 and on the right of the tie rod bolt 13 a non tie rod bolt is disposed the non tie rod bolts including the upper linking bolt 33 and the lower linking bolt 34 are the extending bolts 35 each having the protruding part 37 longer than that of the other tie rod bolt 13 of the upper linking bolts furthermore as shown in fig1 and fig3 a protruding part 38 of the extending bolt 35 has.