A hidden braking device (3) includes a brake handle (31), a brake member (32) controlling a wheel set (B), and a connecting member (33) disposed in a main frame (4) having telescoping upper and lower frames (41, 42) . The connecting member (33) has an upper stem (331) connected to the brake handle (31) and having fixing notches (33120), a lower stem (332) connected to the brake member (32), a locking set (333) disposed on the lower frame (42) and mainly including a locking unit (3332), and an adjusting set (334) connected to both stems (331, 332) . The adjusting set (334) mainly has an operating unit (3342) on which a first spring unit (3346) is disposed, an engaging case (3344) and a positioning case (3343) disposed relative to each other, and an elastic unit (3345) located between both cases (3343, 3344) and engaged with the fixing notches (33120) . The first spring unit (3346) pushes the operating unit (3342) while loosening the locking unit (3332) to thereby position the engaging case (3344) and adjust a full length of the main frame (4) quickly.
