An eyepiece optical member has an easy-to-clean interior, and a head-mounted display device is provided with the eyepiece optical member. The eyepiece optical member includes a housing with a built-in display element, a light guide prism in which image light from the display element enters through an entrance surface and is guided to exit from an exit surface towards an eyeball of an observer so as to be observed as a magnified virtual image, and a protective member formed separately from the housing and covering a portion of the light guide prism. The housing includes two opposing plate-like portions, the light guide prism is fixed to the protective member as an assembled member, and the assembled member is sandwiched between the two opposing plate-like portions. The head-mounted display device includes a support member for fixation to a wearer's head and includes the eyepiece optical member attached to the support member.