A holder unit (30a-30d, 30aA-30dA, 30aB-30dB, 30aC-30dC) comprising: a first holder (31-35, 31A-35A, 31B-35B, 31C-35C) including a first groove portion (100, 100A) that allows a first end of a solar cell module (10, 10B, 10C) to be inserted thereinto in a first direction which corresponds to the direction from the eaves towards the ridge of a roof; and a second holder (31-35, 31A-35A, 31B-35B, 31C-35C) including a second groove portion (200, 200A) that allows a second end of the solar cell module (10, 10B, 10C) opposite to the first end to be inserted thereinto in a second direction opposite to the first direction, wherein the first groove portion (100, 100A) includes a first front portion (101, 101A) facing a first main surface of the solar cell module (10, 10B, 10C), a first back portion (102, 102A, 102B) facing a second main surface behind the first main surface, and a first side portion connecting the first front portion (101, 101A) and the first back portion (102, 102A, 102B) to each other, wherein the second groove portion (200, 200A) includes a second front portion facing the first main surface, a second back portion facing the second main surface (lOss), and a second side portion connecting the second front portion and the second back portion to each other, wherein, in the first groove portion (100, 100A), an end of the first front portion (101, 101A) on the second holder side is located nearer to the first side portion than an end of the first back portion (102, 102A, 102B) on the second holder side, and the first groove portion (100, 100A) has a first gap and a second gap in a third direction from the first front portion (101, 101A) toward the first back portion (102, 102A, 102B), the first gap being a gap between the end of the first front portion (101, 101A) on the second holder side and a part of the first back portion (102, 102A, 102B) facing the end of the first front portion (101, 101A) on the second holder side, the first gap being larger than a thickness of the solar cell module (10, 10B, 10C), the second gap being a gap between the first back portion (102, 102A, 102B) and a part of the first front portion (101,101A) between the first side portion and the end of the first front portion (101, 101A) on the second holder side, wherein the second gap is larger than both of the first gap and a third gap that is smaller than the first gap in the third direction, the third gap being a gap between an imaginary extension surface extending from the first front portion (101, 101A) toward the second holder (31-35, 31A-35A, 31B-35B, 31C-35C) and the end of the first back portion (102, 102A, 102B) on the second holder side, wherein a first depth of the first groove portion (100, 100A) from the end of the first front portion (101, 101A) on the second holder side to the first side portion in the first direction is larger than a second depth of the second groove portion (200, 200A) from the end of the second front portion on the first holder side to the second side portion in the second direction, wherein the second holder (31-35, 31A-35A, 31B-35B, 31C-35C) includes an extending portion and an overhanging portion, the extending portion extending from the end of the second back portion on the first holder side in a direction away from the second front portion, the overhanging portion sticking out toward the first holder (31-35, 31A-35A, 31B-35B, 31C-35C) from a portion of the extending portion excluding both ends of the extending portion in an extending direction, and wherein a distance between the first side portion and the extending portion is set to be a distance allowing the second end to be placed on the overhanging portion in loose contact with the extending portion in a state in which the first end is in contact with the first side portion.