PURPOSE: To enhance the characteristic in temperature and prevent drift by vibrating the vibrator disposed in a fluid to be measured alternately in accordance with a first and a second modes of vibration and obtaining at this time the density of the fluid to be measured, from the number of vibration.

CONSTITUTION: In the case of a switch 16 being connected to an amplifier 14, piezoelectric elements 2W4, charge amplifiers 6, 7, coefficient circuit 8, adding circuit 10, a low path filter 12 and an amplifier 14 constitute a driving portion A for driving a cylindrical trembler 1. In the case of a switch 16 being connected to an amplifier 15, piezoelectric elements 2W4, charge amplifiers 6, 7, a coefficient circuit 9, a subtraction circuit 11, a low path filter 13 and an amplifier 15 constitute a driving portion B for driving a cylindrical trembler 1. Thus, a first and a second vibrating modes makes the vibrator 1 vibrate and from the vibrating number at this time, the density of the member to be measured can be obtained.