An exhaust gas purification apparatus for an internal combustion engine which includes: an exhaust gas purification catalyst that is arranged in an exhaust passage of the internal combustion engine; a tank that stores an additive agent in a solid or a liquid state, which is supplied to said exhaust gas purification catalyst and which acts as a reducing agent or an oxidizing agent; a vaporization device that vaporizes said solid or liquid additive agent by heating the same; and a supply passage that connects between said tank and said exhaust passage at the upstream side of said exhaust gas purification catalyst, with a gaseous additive agent vaporized by said vaporization device flowing therethrough, wherein the gaseous additive agent vaporized by said vaporization device is supplied to said exhaust passage at the upstream side of said exhaust gas purification catalyst; characterized in that said exhaust gas purification apparatus comprises: a dilution device that dilutes said gaseous additive agent with another gas in said supply passage; a metering device that adjusts in said supply passage an amount of the additive agent to be supplied from said supply passage to said exhaust passage after being diluted by said dilution device; and an adjusting device that is arranged in said supply passage at the upstream side of said metering device, for adjusting the temperature or pressure of the additive agent which has been diluted by said dilution device. an exhaust gas purification catalyst that is arranged in an exhaust passage of the internal combustion engine; a tank that stores an additive agent in a solid or a liquid state, which is supplied to said exhaust gas purification catalyst and which acts as a reducing agent or an oxidizing agent; a vaporization device that vaporizes said solid or liquid additive agent by heating the same; and a supply passage that connects between said tank and said exhaust passage at the upstream side of said exhaust gas purification catalyst, with a gaseous additive agent vaporized by said vaporization device flowing therethrough, wherein the gaseous additive agent vaporized by said vaporization device is supplied to said exhaust passage at the upstream side of said exhaust gas purification catalyst; characterized in that said exhaust gas purification apparatus comprises: a dilution device that dilutes said gaseous additive agent with another gas in said supply passage; a metering device that adjusts in said supply passage an amount of the additive agent to be supplied from said supply passage to said exhaust passage after being diluted by said dilution device; and an adjusting device that is arranged in said supply passage at the upstream side of said metering device, for adjusting the temperature or pressure of the additive agent which has been diluted by said dilution device. a dilution device that dilutes said gaseous additive agent with another gas in said supply passage; a metering device that adjusts in said supply passage an amount of the additive agent to be supplied from said supply passage to said exhaust passage after being diluted by said dilution device; and an adjusting device that is arranged in said supply passage at the upstream side of said metering device, for adjusting the temperature or pressure of the additive agent which has been diluted by said dilution device.