High-frame-rate echocardiography using unfocused transmit beams and parallel receive beamforming is a promising method for evaluation of cardiac function, such as imaging of rapid propagation of the heart wall vibration. In this technique, high temporal resolution is realized at the expense of spatial resolution and contrast. The phase coherence factor has been developed to improve spatial resolution and contrast in ultrasonography. It evaluates the variance in phases of echo signals received by individual transducer elements after delay compensation as in the conventional delay and sum beamforming process. However, the phase coherence factor suppresses speckle echoes because phases of speckle echoes fluctuate due to their mutual interference. In the present study, total receiving aperture was divided into several sub-apertures, and conventional delay and sum beamforming was performed with respect to each sub-aperture to suppress echoes from scatterers except for that from a focal point. After sub-aperture beamforming, the phase coherence factor was obtained from beamformed RF signals from respective sub-apertures. By means of this procedure, undesirable echoes, which can interfere with the echo from a focal point, can be suppressed by sub-aperture beamforming, and the degradation of the phase coherence factor due to phase fluctuation caused by such interference can be avoided. Effects of sub-aperture beamforming were evaluated using a phantom. By sub-aperture beamforming, the average intensity of speckle echoes from a diffuse scattering medium was significantly higher (-39.9 dB) than that obtained without sub-aperture beamforming (-48.7 dB). As for spatial resolution, the width at half maximum of the lateral echo amplitude profile obtained without the phase coherence factor was 1.06 mm. Using the phase coherence factor, spatial resolution was improved significantly, and sub-aperture beamforming achieved a better spatial resolution of 0.75 mm than that of 0.78 mm obtained without sub-aperture beamforming. Using sub-aperture beamforming in estimation of coherence factor, better visualization of speckle echoes and spatial resolution could be realized simultaneously by suppressing out-of-focus echoes.