To provide an arm-mounted portable apparatus which is easily attached and detached and is easily adjusted in accordance with the thickness of the arm.

The arm-mounted portable apparatus is composed of a band 20 formed with adjust parts 20a and 20b of fine pitches each having a saw- toothed lengthwise cross section and a rectangle-shaped crosswise cross section, and adjust mechanisms 30 and 40 having saw-toothed engage parts which are pulled in one direction by elastic forces at ordinary time to be engaged with the adjust parts 20a and 20b of the band 20 and are pushed in oppositely against the elastic forces by an operating part at adjusting time to be disengaged from the adjust parts 20a and 20b of the band 20. The adjust part is formed on the rear side of the band 20 while inverting the inclining direction of sawteeth approximately with the middle as a boundary. Besides, the adjust mechanism is composed of the first adjust mechanism 30 mounted at the position of 12 o'clock on the case of the portable apparatus and the second adjust mechanism 40 mounted at the position of 6 o'clock thereon.