PURPOSE: To measure density without any variation in liquid level by providing a diffuser below the lower measurement limit of the liquid level in a vaporiza tion can, sending the liquid out of the vaporization can be a circulation pump and letting part of the circulating liquid flow in the diffuser, and measuring the differential pressure by the diffuser.

CONSTITUTION: The liquid in the vaporization can 4 is sent out by the circulation pump 1 and returned to the vaporization can 4, where it is vaporized. Further, a branch pipe 2 is provided at the exit of the circulation pump 1 and the diffuser 3 is provided at a level below the lower measurement limit of the liquid level in the vaporization can. The liquid is run through the branch pipe 2 and diffuser 3 and returned to the vaporization can 4. At this time,the liquid run through the diffuser 3 is reduced in flow rate to suppress the influence of dynamic pressure and the pressure difference of the liquid is detected by a differential pressure gauge 5. The differential pressure is therefore measured without being affected by variation in the liquid level in the vaporization can, so the concentration is measured accurately.