To provide a fluid side fluid measuring instrument for measuring a concentration of a fluid, a density thereof, a level or other physical property of the fluid.

This fluid measuring instrument is used in a fluid vessel for storing the fluid, and includes a logical operation unit, an electrostatic capacitance type detecting element, and an electrostatic capacitance detecting circuit. The capacitance detecting circuit detects an electrostatic capacitance value of the capacitance type detecting element, through an input of a constant current, and converts the electrostatic capacitance value into a physical feature message through the logical operation unit. The physical feature message includes any of fluid physical features of the concentration, density and level of the fluid, In addition, the capacitance type detecting element includes an electrostatic capacitance element serving as a standard, and further an electrostatic capacitance element for detecting actually the fluid, and obtains an electrostatic capacitance value in the latter through a differential circuit of the capacitance detecting circuit.