PURPOSE: To detect the mass of ink particles with high accuracy in a short time by applying charges to ink particles injected, deflecting the direction of ink particles in a deflecting section, receiving the direction of ink particles in a recording section, measuring the quantity of deflection and detecting the mass of ink particles in the title device of ink-jet head.

CONSTITUTION: An applying section 3 and a deflecting section 4 are disposed in series with the injection path 10 of ink particles 2 injected from an ink-jet head 1. Ink particles 2 are passed through the center of the space S of the electrodes 3A of the applying section 3, and a fixed quantity of charges are applied to ink particles 2. Ink particles 2, to which charges are applied, are passed between deflecting plates 4A forming a electromagnetic field 5 and deflected, and recorded as a point P1 on the recording paper 6A of a recording section 6. Likewise, the applied voltage of the applying section 3 is fluctuated, and a point P2 is recorded. A distance between the points P1, P2 is measured and the quantity of deflection D is acquired, and the mass of ink particles are obtained from the electric field G of the electromagnetic field 5, jet velocity, the length L of the deflecting plates 4A and a distance (b) between the deflecting plates 4A and recording paper 6A.