A specific number of adjacent pixels are combined into blocks. The tone values of the pixels in a target block are detected when the dot on-off state is determined, and the fact that the target block does or does not satisfy specific processing conditions is confirmed based on the relation between the magnitudes of the tone values thus detected. The dot on-off state is determined for the target block in block units if the target block satisfies the processing conditions. The conversion procedure can be rapidly performed by adopting this approach. The dot on-off state is determined for the pixels of the target block if the processing conditions are not satisfied. Image quality is thereby prevented from being adversely affected. Image quality can thus be preserved and image data rapidly converted to a specific expression format based on the dot on-off state by performing the conversion procedure in accordance with an appropriate method selected based on whether the processing conditions are satisfied.