A fluid-pressure indicating apparatus 10 includes a mounting mechanism which can be simply removed from a rail 11 when the apparatus is used alone but which cannot be removed from the rail when the apparatus is coupled to adjacent apparatuses. The apparatus 10 includes recessed portions 17a and 17b formed in opposite walls of a rail mounting groove 16. An elastic piece 18, which defines the depth of one of the recessed portions 17a is provided to the side of the recessed portion 17a in such a way as to be elastically movable. A stopper 19 is also provided to the side of the first recessed portion 17a which engages the elastic piece 18 of an adjacent fluid-pressure indicating apparatus so as to control its movement when the apparatus is coupled to the adjacent fluid-pressure indicating apparatus.