Provided are a protecting device capable of safely and quickly interrupting a current path by restricting heat absorption to a lower case, and a battery pack using the same. A protecting device includes: a meltable conductor 3; and a housing 6 including a lower case 4 and an upper case 5, the housing being formed by joining the lower case 4 and the upper case 5, and the lower case 4 is provided with a recessed portion 23 having support portions 21 provided at opposing side edges of the recessed portion 23 and hollow portions 22 provided on the side edges substantially orthogonal to the side edges of the recessed portion 23 on which the support portions 21 are provided.