An object of the present invention is to provide an objective optical system and a photoacoustic imaging apparatus capable of obtaining a clearer image of a sample than before. The objective optical system (23) includes: a convex mirror (101) having a convex reflecting surface for reflecting pulsed light traveling toward a sample (SP); a concave mirror (102) having a concave reflecting surface for reflecting the light reflected by the convex mirror (101) and irradiating the sample (SP) with the light; and an ultrasonic detector (103) having at least one end portion provided on an object side of the convex mirror (101), and detecting an acoustic wave obtained by irradiating the sample (SP) with the light.