A printing unit for gravure printing a sheet or reel material, comprising: - an impression roller comprising an impression roller body and an impression roller sleeve fitted, or capable of being fitted, onto said impression roller body, said impression roller body defining an impression roller rotation axis and an impression roller body side surface; said impression roller sleeve having a tubular shape defining an outer sleeve side surface and an inner sleeve side surface, said impression roller sleeve comprising an outer layer made of elastomeric material delimited by said outer sleeve side surface; - a checking apparatus for measuring the electrical insulation resistance of said impression roller between said inner sleeve side surface and said outer sleeve side surface, said printing unit being characterized in that said checking apparatus is permanently associated with said printing unit, and comprises: - an electrical contact element; - a movement device for moving said electrical contact element between a measuring position, in which the electrical contact element is in contact with the outer sleeve side surface, and a rest position, in which said electrical contact element is not in contact with, and spaced apart from, the outer sleeve side surface; - an electrical circuit connected to said electrical contact element and capable of being connected to said inner sleeve side surface, when said sleeve is fitted onto said impression roller body, so that when the electrical contact element is in contact with the outer sleeve side surface, said electrical circuit is closed through said impression roller sleeve between said inner sleeve side surface and said outer sleeve side surface; said electrical circuit being configured to provide a measured electrical insulation resistance value of the sleeve between said inner sleeve side surface and said outer sleeve side surface. - an impression roller comprising an impression roller body and an impression roller sleeve fitted, or capable of being fitted, onto said impression roller body, said impression roller body defining an impression roller rotation axis and an impression roller body side surface; said impression roller sleeve having a tubular shape defining an outer sleeve side surface and an inner sleeve side surface, said impression roller sleeve comprising an outer layer made of elastomeric material delimited by said outer sleeve side surface; said impression roller body defining an impression roller rotation axis and an impression roller body side surface; said impression roller sleeve having a tubular shape defining an outer sleeve side surface and an inner sleeve side surface, said impression roller sleeve comprising an outer layer made of elastomeric material delimited by said outer sleeve side surface; - a checking apparatus for measuring the electrical insulation resistance of said impression roller between said inner sleeve side surface and said outer sleeve side surface, said printing unit being characterized in that said checking apparatus is permanently associated with said printing unit, and comprises: - an electrical contact element; - a movement device for moving said electrical contact element between a measuring position, in which the electrical contact element is in contact with the outer sleeve side surface, and a rest position, in which said electrical contact element is not in contact with, and spaced apart from, the outer sleeve side surface; - an electrical contact element; - a movement device for moving said electrical contact element between a measuring position, in which the electrical contact element is in contact with the outer sleeve side surface, and a rest position, in which said electrical contact element is not in contact with, and spaced apart from, the outer sleeve side surface; - an electrical circuit connected to said electrical contact element and capable of being connected to said inner sleeve side surface, when said sleeve is fitted onto said impression roller body, so that when the electrical contact element is in contact with the outer sleeve side surface, said electrical circuit is closed through said impression roller sleeve between said inner sleeve side surface and said outer sleeve side surface; said electrical circuit being configured to provide a measured electrical insulation resistance value of the sleeve between said inner sleeve side surface and said outer sleeve side surface.