A specific gravity detector includes an electromechanical vibrator having a central axis and capable of vibrating about the central axis, a transmission shaft having one end thereof connected coaxially to the vibrator, and a detector member connected coaxially to the other end of the transmission shaft. The detector member is immersed in liquid, the specific gravity of which is to be measured and vibrated about the central axis of the vibrator within the liquid by the vibration of the vibrator transmitted through the transmission shaft to measure the specific gravity of the liquid based on a variation in the resonance frequency of the vibrator corresponding to the moment of inertia of the liquid.