A brush unit comprising: a front surface ; a plurality of tooth cleaning elements coupled to and extending from the front surface in a pressing direction, the tooth cleaning elements forming a bristle field on the front surface ; a light transmitting part formed on the front surface adjacent the bristle field; and a light receiving part formed on the front surface adjacent the bristle field; characterized in that : the light receiving part is formed on an opposite side of the bristle field from the light transmitting part ; and the light transmitting part is configured to direct light transmitted therethrough in a first direction, the first direction intersecting the pressing direction and lying within a plane including both the light transmitting part and the light receiving part ; wherein the cross-sectional shape of a first transmission member , which forms part of the light transmitting part , has a side wall running in a direction orthogonal to the pressing direction of the tooth cleaning elements and extends in a slanted angle toward the light receiving part , and wherein the cross-sectional shape of a second transmission member which forms part of the light receiving part , has a side wall running in a direction orthogonal to the pressing direction of the tooth cleaning elements and extends in a slanted angle toward the light transmitting part . a front surface ; a plurality of tooth cleaning elements coupled to and extending from the front surface in a pressing direction, the tooth cleaning elements forming a bristle field on the front surface ; a light transmitting part formed on the front surface adjacent the bristle field; and a light receiving part formed on the front surface adjacent the bristle field; characterized in that : the light receiving part is formed on an opposite side of the bristle field from the light transmitting part ; and the light transmitting part is configured to direct light transmitted therethrough in a first direction, the first direction intersecting the pressing direction and lying within a plane including both the light transmitting part and the light receiving part ; wherein the cross-sectional shape of a first transmission member , which forms part of the light transmitting part , has a side wall running in a direction orthogonal to the pressing direction of the tooth cleaning elements and extends in a slanted angle toward the light receiving part , and wherein the cross-sectional shape of a second transmission member which forms part of the light receiving part , has a side wall running in a direction orthogonal to the pressing direction of the tooth cleaning elements and extends in a slanted angle toward the light transmitting part . the light receiving part is formed on an opposite side of the bristle field from the light transmitting part ; and the light transmitting part is configured to direct light transmitted therethrough in a first direction, the first direction intersecting the pressing direction and lying within a plane including both the light transmitting part and the light receiving part ; wherein the cross-sectional shape of a first transmission member , which forms part of the light transmitting part , has a side wall running in a direction orthogonal to the pressing direction of the tooth cleaning elements and extends in a slanted angle toward the light receiving part , and wherein the cross-sectional shape of a second transmission member which forms part of the light receiving part , has a side wall running in a direction orthogonal to the pressing direction of the tooth cleaning elements and extends in a slanted angle toward the light transmitting part . <EOS>