Radiation sensitive resin composition, pattern forming method and onium salt compound

The present invention provides: a radiation sensitive resin composition which is capable of exhibiting sensitivity, CDU performance and LWR performance at sufficient levels; and a method for forming a resist pattern. A radiation sensitive resin composition which contains an onium salt compound, a resin that contains a structural unit having an acid cleavable group, and a solvent, wherein the onium salt compound is at least one compound that is selected from the group consisting of an onium salt compound (1) represented by formula (1) and an onium salt compound (2) represented by formula (2). (In formula (1), R1 represents a hydrogen atom or a monovalent organic group having from 1 to 40 carbon atoms; each of R2 and R3 independently represents a hydrogen atom, a halogen atom, a carboxy group, an amino group or a monovalent organic group having from 1 to 40 carbon atoms, or alternatively, R2 and R3 combine with each other to form a ring structure having from 5 to 8 ring members together with two carbon atoms to