Organometal halide perovskite has emerged as a promising material for solar cells and optoelectronics. Although the long diffusion length of photo-generated carriers is believed to be a critical factor responsible for the material's high efficiency in solar cells, a direct study of carrier transport over long distances in organometal halide perovskites is still lacking. We fabricated highly oriented crystalline CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) thin film lateral transport devices with long channel length (~ 120 $\mu$m). By performing spatially scanned photocurrent imaging measurements with local illumination, we directly show that the perovskite films prepared here have very long transport lengths for photo-generated carriers, with a minority carrier (electron) diffusion length on the order of 10 $\mu$m. Our approach of applying scanning photocurrent microscopy to organometal halide perovskites may be further used to elucidate the carrier transport processes and vastly different carrier diffusion lengths (~ 100 nm to 100 $\mu$m) in different types of organometal halide perovskites.