NOVELTY - An adhesive composition comprises polyisobutylene resin (A), hydrogenated petroleum resin (B), an olefin polymer (C) having 0.3 or more double bonds per molecule and a silane compound (D) having 2 or 3 hydrosilyl groups. The polymer (C) comprises a polyisobutylene resin (C1) having a polymerizable double bond at the terminal and a copolymer (C2) comprising a structural unit derived from norbornene compound (I) or (II), ethylene and propylene. The content of polymer (C) in the adhesive composition is 10 pts. wt. or more with respect to 100 pts. wt. of resin (A).    USE - Adhesive composition is used for adhesive sheet for sealing organic electroluminescent element (claimed). Uses include but are not limited to liquid crystal display device, electronic paper, thin-film transistor, electrochromic device, electrochemiluminescence device, touch panel, solar cell, thermoelectric conversion device and piezoelectric transducer device.    ADVANTAGE - The adhesive composition having excellent adhesive force and retention strength provides an adhesion layer for adhesive sheet. The adhesion layer has excellent transparency, reduced outgas generation and water-vapor transmission rate.    DETAILED DESCRIPTION - An adhesive composition comprises polyisobutylene resin (A) having a weight average molecular weight of 20000-800000, a hydrogenated petroleum resin (B) having 90-135 degrees C softening point, an olefin polymer (C) having 0.3 or more double bonds per molecule and a silane compound (D) having 2 or 3 hydrosilyl groups. The polymer (C) comprises a polyisobutylene resin (C1) having a polymerizable double bond at the terminal and a copolymer (C2) comprising a structural unit derived from a norbornene compound of formula (I or II), ethylene and propylene. The content of polymer (C) in the adhesive composition is 10 pts. wt. or more with respect to 100 pts. wt. of resin (A).    R1=H or optionally substituted 1-5C alkyl;    R2-R4=H or optionally substituted 1-10C alkyl;    p=0-10;    R5-R7=H or optionally substituted 1-10C alkyl;and    or R2 or R3+R4,R5 or R6+R7=ring having double bond.    An INDEPENDENT CLAIM is included for adhesive sheet (1), which contains an adhesive layer (12) comprising adhesive composition.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of adhesive sheet.    Adhesive sheets (1,1a,1b,1c,1d)    Base material (11)    Adhesion layers (12,12')    Peeling materials (13,13')