To provide a simple combustion efficiency improvement device capable of improving combustion efficiency of an internal combustion engine such as an automobile engine, without using large equipment such as a high voltage power supply unit.

A combustion chamber of liquid fuel includes: a first coil wound around an outer circumference of a feed pipe for feeding and injecting the liquid fuel, and a reduction-type modulated electromagnetic field generator for flowing, in the first coil, alternating current of which frequency changes with time in a bandwidth between 4 kHz and 1 MHz. Preferably, the combustion chamber of liquid fuel further includes a second coil wound around an outer circumference of an intake pipe for feeding combustion air for the liquid fuel. Preferably, the first coil and the second coil are made of an uninterrupted conducting wire and share the reduction-type modulated electromagnetic field generator.