To increase reliability of an auxiliary contact unit. An auxiliary contact unit (1) including: a pair of fixed contact pieces (3A, 4A); a movable contact piece (5A) arranged in a contactable and separable manner to and from the pair of fixed contact pieces (3A, 4A) ; an auxiliary contact support (10) configured to hold the movable contact piece (5A) and move in conjunction with opening/closing operation of an electromagnetic contactor (50); and a unit case (20) configured to house the pair of fixed contact pieces (3A, 4A), the movable contact piece (5A), and the auxiliary contact support (10) in a housing portion (24) formed with joining portions (21a, 22a) of respective ones of a first case member (21) and a second case member (22) opposed to each other in a direction orthogonal to the direction of motion (Z-direction) of the auxiliary contact support (10), wherein the joining portions of the respective ones of the first and second case members (21, 22) are air-tightly joined to each other.