In this paper, we propose a novel deep learning framework, Spatio-Frequency Network (SFNet), based on 3D MRI for early diagnosis of Alzheimer's disease (AD). SFNet improves the accuracy of AD diagnosis by simultaneously utilizing information in both spatial and frequency domains. Unlike previous studies that utilized only one of the spatial or frequency domains or were limited to 2D MRI, SFNet is the first end-to-end deep learning model that utilizes both spatial and frequency information of 3D MRI. It extracts local spatial features through an enhanced Dense Convolutional Network, captures global frequency-domain representations through a global frequency module, and improves spatial feature extraction through a multi-scale attention module. Experimental results using the ADNI dataset show that SFNet achieves higher accuracy (95.1%) and reduces computational costs compared to existing methods.