In this paper, the influence of the intertrack interference (ITI) at writing in shingled magnetic recording (SMR) as well as at reading is examined using a read/write model based on recording on a medium composed of discrete Voronoi diagram by a triangular writer with trailing and one-side shields. Two kinds of ITI's at writing which affect the magnetic grains on the track currently being written and the previously recorded track are shown. One is that a part of signal on the previously recorded track remains on the current track due to lack of writing ability of writer. Another is that a part of signal is erased by the next-track writing. The effects are caused from the adjacent tracks at writing. Additionally the ITI at reading deteriorates the read-back waveform. Consequently these ITI's bring the decoding errors. The results by computer simulation using above R/W model show that the decoding errors occur in the low density parts of track which were originally considered to be hard to make errors.