We propose a method of document image retrieval using digital cameras. The proposed method takes as input a part or the whole of a document acquired as a query by a digital camera, and retrieves a document image that includes the query. For this purpose, it is required to solve the problem of "perspective distortion" of images, as well as to establish a way of matching parts of document images flexibly. These are achieved based on the following characteristics of the proposed method: (1) Indexing of document images using the projective invariants called the "cross-ratios", (2) Retrieval as voting for partial signatures of document images defined by the cross-ratios. From experimental results using digital cameras with high and low resolutions, we demonstrate the effectiveness of the proposed method.