A great deal of interest is focused on Homogeneous Charge Compression Ignition (HCCI) combustion today as a combustion system enabling internal combustion engines to attain higher efficiency and cleaner exhaust emissions. Because the air-fuel mixture is compression-ignited in an HCCI engine, control of the ignition timing is a key issue. Additionally, because the mixture ignites simultaneously at multiple locations in the combustion chamber, it is necessary to control the resultant rapid combustion, especially in the high-load region. Supercharging can be cited as one approach that is effective in facilitating high-load operation of HCCI engines. Supercharging increases the intake air quantity to increase the heat capacity of the working gas, thereby lowering the combustion temperature for injection of the same quantity of fuel. In this study, experiments were conducted to investigate the effects of supercharging on combustion characteristics in an HCCI engine. Light emission and absorption spectroscopic measurement techniques were used to investigate the combustion behavior in detail. Chemical kinetic simulations were also conducted to analyze the reaction characteristics in detail. The results made clear that the raising the intake air pressure under a condition of a constant quantity of heat produced per cycle by the injected fuel has the effect of reducing the combustion temperature. As a result, combustion becomes more moderate. The results of spectroscopic measurement of light absorption also showed a sharp rise in absorbance at 293.1 nm, which is attributed to formaldehyde (HCHO) produced by the low-temperature reactions. Subsequently, absorbance dropped sharply at the time autoignition occurred.