A low-cost sheet thickness detection device has high sheet follow-up capability, has no influence on the conveyance of a sheet of paper, receives no influence of vibration and the like when mounted in a central position of the sheet in the width direction thereof, and allows complicated operations to be avoided when the conveying path is to be opened, and provides a sheet conveying device and an image forming apparatus. The sheet thickness detection device is provided in a sheet conveying path formed by a pair of conveyance guides opposite to each other to detect the thickness of a sheet conveyed in the sheet conveying path. It includes a pair of levers which are mounted on the pair of conveyance guides, respectively, and each of which is swingably pivoted at one end thereof to allow opening and closing motions of another end thereof about the end serving as a base point.