A friction-coefficient estimating device is configured to estimate friction coefficient of the surface of a medium in a form of a sheet by irradiating a light on the surface and by detecting specularly-reflected light component of a reflected light and a diffusely-reflected light intensity. The friction-coefficient estimating device comprises an irradiating unit including a first irradiating unit and a second irradiating unit, a specularly-reflected light receiving unit including a first specularly-reflected light receiving unit that receives a first specularly-reflected light component of a reflected light and detects a first specularly-reflected light intensity and a second specularly-reflected light receiving unit that receives a second specularly-reflected light component of a reflected light and detects a second specularly-reflected light intensity, and a diffusely-reflected light receiving unit that receives a diffusely-reflected light component of a reflected light and detects a diffusely-reflected light intensity, and a control unit that estimates a friction coefficient of the surface based on a first reflected-light intensity coefficient and a second reflected-light intensity coefficient.