PURPOSE: To measure the ratio of powder content in a 2-phase fluid at a high accuracy in a wide range by performing a computation with an electronic circuit according to a specified empirical formula.

CONSTITUTION: A voltage V developing at the output end of an electrostatic capacity detection circuit 1 connected to an electrostatic electrode which presents an electrostatic capacity C0 during the air-core period is introduced into an addition circuit 2 and the voltage value F as given below corresponding to the denominator at the second term in the equation I preset separately is added thereto to compute (V+F). F=KC0(1A-1+1)0. The addition outut voltage (V+F) is introduced to an inverse number circuit 3 to determine the inverse number of the voltage value while a computation of G/X is done by multiplying the results by a coefficient G represented by the equation as given below. G=γc 1KC0A0. Then, the computation output voltage G/X of the inverse number circuit 3 is introduced to a subtraction circuit 4 to be subtracted from a voltage corresponding to the true specific gravidy γc of a powder to measure the concentration thereof. This enables the extraction of a voltage directly proportional to the concentration ρ of the powder according to the equation as subtraction output voltage.