We propose an interaction technique called "gripping" to improve the operation of input with pen-based interfaces. This operation involves a strong grip when holding a pen. It can provide new input operations while maintaining the usability of the pen because users can grip it without having to lift their fingers off it. There is an interaction technique called pen pressure that users can use without having to lift their fingers off the pen. We introduce a novel interaction technique that combines gripping and pen pressures. This combination enables users to simultaneously input new two values in addition to common pen interactions like those with tapping or stroking. By applying this technique to a paint tool, for instance, novel drawing operations become possible and artists can create new artworks. Two experiments were conducted to investigate whether users could perform an operation that combined gripping and pen pressures. As a result, they confirmed that combined interaction was possible and there was an optimal range of pressures that enabled combined interaction. We also implemented an application software for this combined interaction.