As part of my study of the universe, I am observing type Ia supernovae (SNe Ia). An SNe Ia type surpernovae is thought to be the thermonuclear explosion of a white dwarf that has exceeded 1.4 solar masses as a result of accreting mass from its companion star in a binary system. SNe Ia have similar luminosity and spectrum characteristics, as well as a maximum luminosity comparable to that of an entire galaxy. Assuming that SNe Ia have a constant absolute luminosity, we can determine their distance up to 9 billion light-years, which gives us information about the expansion history of the universe and the content of its dark matter and dark energy. I have searched for distant SNe Ia using the NAOJ’s Subaru telescope in collaboration with the Supernova Cosmology Project and found this method to be more effective than the Hubble Space Telescope for detecting distant SNe Ia. The data has been used to measure the SNe Ia rate and constrain the progenitor system for SN Ia. I also joined the SDSS-II Supernova Survey and have obtained about 500 multi-band light curves from SNe Ia. This data has been used to examine the relationship between SNe Ia and their host galaxies.