Abstract: A servo system includes a driven body (C), a hydraulic driving unit (A) for moving the driven body (C), a mechanical hydraulic servo valve (1), a sheave (31), a flexible measuring tape (33), a rotary driving source (M) and an adjusting unit (4,5,6,7). The servo valve (1) includes a spring-biased movable spool (11) and can activate the driving unit (A) to move the driven body (C) when the spool (11) is moved. The sheave (31) is mounted on the driven body (C) and has an outer peripheral surface. The measuring tape (33) has a first end portion secured to the sheave (31), a second end portion (331), and an intermediate portion wound on the sheave (31) along the outer peripheral surface of the sheave (31). The rotary driving source (M) is coupled operably to the sheave (31) and is operable to rotate the sheave (31) in order to wind and unwind the measuring tape (33).