A liquid level detecting apparatus of a tank for a vehicle is provided. The apparatus includes a main detecting part that detects the liquid level of the liquid in a tank to output a liquid level detection signal corresponding to the liquid level and an auxiliary floater assembly that has an auxiliary floater provided to float on the liquid surface of the liquid in the tank. A gyro sensor is mounted to the auxiliary floater, and an angle detection signal corresponding to an inclined angle of the tank is output by the gyro sensor. A controller determines a final liquid level value by correcting a liquid level value obtained from the liquid level detection signal based on the angle detection signal.