To provide an ionization detector having a good S/N ratio by performing a trace detection.

The ionization detector comprises upper and lower ionization chambers 114, 116, and a radioactive particle source 140. In this case, a gas to be detected is charged in the chamber 114. The radiation source is disposed on a periphery of the chamber 14 to supply radioactive particles, thereby generating metastable atoms and photons by an interaction of the gas. The chamber 116 is coupled to the chamber 114 to receive the gas to be detected and the atoms. A sample fluid is introduced into the chamber 116 together with a carrier gas flow, and the atoms ionize analytical material molecules. An exit is provided at a base of the chamber 116 to discharge a bonding flow of these gases. And, the chamber 114 limits a radioactive particle route via a radioactive particle barrier inserted into a route between the source and the chamber 116 to prevent it from invading into the chamber 116.