A monitoring device including a microcomputer of a detector that calculates a first time derivative value by differentiating a first pressure value with respect to time, and/or calculates a second time derivative value by differentiating a second pressure value with respect to time. In addition, based on at least one from among the first time derivative value and the second time derivative value, the microcomputer determines whether or not the piston has arrived at one end or another end inside the cylinder main body.