Recently an in vivo high-resolution backward-mode photoacoustic microscope was developed that shows potential for applications in dermatology and related cancer research. However, the limited depth of focus of the large-numerical-aperture (NA) ultrasonic lens employed in this system causes the image quality to deteriorate significantly in the out-of-focus region. To solve this problem, we devised and explored, for the first time to our knowledge, a virtual-detector-based synthetic-aperture focusing technique, combined with coherence weighting, for photoacoustic microscopy with such a large-NA transducer. Images of phantoms show that the proposed technique improves the $-6\mathrm{dB}$ lateral resolution from $49–379\text{to}46–53\mu \mathrm{m}$ and increases the signal-to-noise ratio by up to $29\mathrm{dB}$ , depending on the distance from the ultrasonic focal point. In vivo experiments show that the technique also provides a clearer representation of the vascular distribution in the rat’s scalp.