The explosive self-decomposition characteristics of gaseous ozone with a concentration of up to almost 100vol% were quantitatively investigated using a closed system with an electric spark device. The lower self-decomposition (explosion) limit for ozone diluted with oxygen at room temperature and atmospheric pressure was 10–11vol%, and so ozone at more than 10–11vol% will lead to an explosive chain decomposition reaction leading to its complete conversion to oxygen in a vessel. The lower explosion limit shifts to a higher concentration with a decrease in pressure. The limit was about 80vol% under a reduced pressure of 10 Torr. We also confirmed that explosion trigger energy (minimum ignition energy) is strongly dependent on ozone concentration and pressure. For example, the minimum trigger energy for 15vol% ozone at a pressure of 76 Torr (about 220mJ) was more than 20-fold that at atmospheric pressure (about 10mJ), and that for 13vol% ozone (about 580mJ) was approximately 30 times higher than that for 20vol% (about 20mJ) at the same pressure of 76 Torr. Moreover, the physical characteristics of the trigger energy sources (e.g. spark gap and electrode tip angle) leading to the decomposition (explosion) of ozone were investigated under various conditions.