A housing 12 is connectable to a mating connector, and has inwardly-projecting housing bosses 41a and 41b. A lever 2 includes engagement portions 21a and 21b engageable respectively with the housing bosses 41a and 41b, and a lever boss 21e for retaining engagement with a retaining portion formed on the mating connector. Guide portions 21c and b 21d for guiding the insertion of the lever 2 into the housing 12 are formed at the lever 2, and a relief portion 41c for the passage of the lever boss 21e therethrough during the insertion is formed at the inside of the housing 12. At the time of starting the insertion of the lever 2 into the housing 12, the distance between each guide portion 21c, 21d and the corresponding housing boss 41a, 41b is smaller than the distance between the lever boss 21e and the relief portion 41c.