PURPOSE: To obtain a liquid-amount detection apparatus, for a tank, in which a failure rate is lowered, whose maintainablity is enhanced and which is labor-saving and economical by a method wherein an air amount inside the tank is detected by a loudspeaker sound source attached to a liquid injection pipe and by a microphone detecting a reverberation sound inside the tank, a liquid amount inside the tank is found by a computing operation on the basis of it and a moving part used to detect a liquid surface or a part coming into direct contact with a liquid is eliminated.

CONSTITUTION: A liquid-amount detection apparatus detects a liquid amount inside a container 1 at which a pipe 2 having a prescribed cross section has been installed. The liquid-amount detection apparatus is provided with a loudspeaker 3 which is installed at the inside of the pipe and which generates sound waves while a frequency is being changed, with a microphone 4 which is installed additionally at the inside of the pipe and which detects the maximum level of a reververation sound generated by the loudspeaker and with a processor 5 which detects the liquid amount inside the container by Helmholtz's resonance principle on the basis of the frequency of the sound waves generated by the loudspeaker when the reververation sound has reached the maximum level.