The present invention provides an optical unit and an imaging device, which are capable of providing a small-sized low-cost imaging optical system that has high resolution and high heat resistance. The optical unit comprises at least a first lens group (110) and a second lens group (120) among the first lens group, the second lens group and a third lens group which are sequentially arranged from the object side to the image plane side. The first lens group (110) comprises an assembly of at least a first lens element (111) and a second lens element (112), a transparent body (113) and a third lens element (114) which are sequentially arranged from the object side to the image plane side. The first lens element (111) and the second lens element (112) of the assembly are formed from resin materials that have different temperature-dependent refractive index changes.