To provide a pipet for automatically preparing a fluid only by an accurate quantity.

The flow of the fluid is measured indirectly using a non-linear system model correlated with the vacuum present in the upper part of the interrupted liquid column in the pipet. The sensing of the flow of the fluid required in the case of a contact type closed loop is eliminated by non-contact type operation and a risk of mutual contamination accompanying it is also eliminated. In one embodiment, the open time of a valve is calculated dynamically on the basis of a non-linear system by the electronic controller in a pistol-shaped wireless self-completion type pipetter case. Calibration is used in order to draw out a constant of a formula and, in order to accurately prepare a programmed desired amount of the fluid, the pressure of the vacuum of the liquid column before the valve is opened is used as an independent variable for drawing out the open time of the valve. By a relatively inexpensive portable pipet or device, repeated preparation can be automatically performed in an accuracy of 1% or above without requiring a mechanistically complicated volume type one.