To provide a mass measuring method which can accurately measure the mass of a droplet or a solid in the droplet dripped by an ink jet system, and a mass measuring apparatus.

In a mass measuring method, a droplet to be measured is dripped from an IJ head (12) to a crystal oscillator (14) that is oscillated, resonance frequencies F₁, F₂ before and after depositing the droplet to be measured to the crystal oscillator are measured and the mass of the droplet or a solid contained in the droplet is calculated on the basis of a difference of F₁, F₂. Total volume V of droplets to be measured represented with a surface area A_Q of an electrode (42) of the crystal oscillator, a resonance frequency F in the case where no droplet is deposited, viscosity η of the droplet and a density ρ of the droplet satisfies V≤A_Q×{η/(π×ρ_L×F)}^1/2. After droplets to be measured are not matched and the preceding droplet reaches an equilibrium state, the next droplet is dripped, and the resonance frequency F₁ before depositing the droplet to be measured is measured. After the preceding droplet reaches the equilibrium state, the resonance frequency F₂ after depositing that droplet is measured.