A connector unit comprising: a first housing including a mating portion; and a connector comprising: a second housing configured to be attached to the first housing; and a seal member configured to be attached to the second housing, wherein the second housing includes a groove portion; and wherein the groove portion is configured such that the seal member is attached to the groove portion and extends continuously over a circumferential direction of the second housing, the groove portion including a bottom surface, a first side wall provided in a first direction in which the second housing is attached to the first housing, and a second side wall provided in a second direction being opposite to the first direction; and wherein the second housing includes a fitting portion configured such that the mating portion is attached to the fitting portion, characterized in that the second housing further includes a protruding portion that extends from the second side wall toward the first side wall and has an end surface and an outer circumferential surface; the end surface of the protruding portion faces the first side wall; the outer circumferential surface connects the end surface of the protruding portion and the second side wall, a part of the outer circumferential surface having an inclined surface inclined such that a first end of the inclined surface in the first direction is closer to the bottom surface than a second end of the inclined surface in the second direction; the seal member includes an engaging portion configured to, when the seal member is attached to the groove portion, engage with the protruding portion; the engaging portion and the protruding portion are configured to be engaged with each other and to prevent the seal member from moving in the circumferential direction with respect to the second housing; an end portion of the mating portion of the first housing is configured to, when the first housing and the second housing are attached to each other, face the outer circumferential surface; and at least a part of the end surface of the protruding portion has a second inclined surface inclined such that an outer end of the second inclined surface in a radial direction of the second housing is closer to the first side wall than an inner end of the second inclined surface in the radial direction. a first housing including a mating portion; and a connector comprising: a second housing configured to be attached to the first housing; and a seal member configured to be attached to the second housing, wherein the second housing includes a groove portion; and a second housing configured to be attached to the first housing; and a seal member configured to be attached to the second housing, wherein the second housing includes a groove portion; and wherein the groove portion is configured such that the seal member is attached to the groove portion and extends continuously over a circumferential direction of the second housing, the groove portion including a bottom surface, a first side wall provided in a first direction in which the second housing is attached to the first housing, and a second side wall provided in a second direction being opposite to the first direction; and wherein the second housing includes a fitting portion configured such that the mating portion is attached to the fitting portion, characterized in that the second housing further includes a protruding portion that extends from the second side wall toward the first side wall and has an end surface and an outer circumferential surface; the end surface of the protruding portion faces the first side wall; the outer circumferential surface connects the end surface of the protruding portion and the second side wall, a part of the outer circumferential surface having an inclined surface inclined such that a first end of the inclined surface in the first direction is closer to the bottom surface than a second end of the inclined surface in the second direction; the seal member includes an engaging portion configured to, when the seal member is attached to the groove portion, engage with the protruding portion; the engaging portion and the protruding portion are configured to be engaged with each other and to prevent the seal member from moving in the circumferential direction with respect to the second housing; an end portion of the mating portion of the first housing is configured to, when the first housing and the second housing are attached to each other, face the outer circumferential surface; and at least a part of the end surface of the protruding portion has a second inclined surface inclined such that an outer end of the second inclined surface in a radial direction of the second housing is closer to the first side wall than an inner end of the second inclined surface in the radial direction.