PROBLEM TO BE SOLVED: To provide a composite filter medium causing no breakage even if subjected to pleating work and having a high collection efficiency in a filter medium formed by holding gas-removing particles, and an air filter unit formed by pleating the filter medium.SOLUTION: The composite filter medium is formed by holding the gas-removing particles between at least two base material layers containing electret melt blown nonwoven fabric sheet, with a larger value of strength in 2% elongation of the electret melt blown nonwoven fabric sheet in the longitudinal direction or lateral direction of 0.03-0.4 MPa.
